[JohnBax]

I have a Sony A77 ii and I am confused about the relationship between sensor size and the F rating for a lens. I understand the crop factor for the lens size i.e.. a 50mm lens is the equivalent of a 75mm but if the lens is a f2.8 should the f no be multiplied by the crop factor? Does this mean I am wasting money on fast lenses and are the manufacturers' giving wrong information?

[EnglishBob]

The F Stop just effects the amount of light coming in, and the DoF of course. A Smaller sensor does capture less of that light than a full frame would, howeverit still benefits from the extra light a larger hole lets in.

I wouldn't day it's wasting money to buy fast lenses, as they are still superior to the slower lens you would use instead. And you don't multiply by the same number as you would for focal length.

Tony Northrup has a very detailed video on F Stop and sensor size on YouTube.

[Rolf]

Hi, Here are a couple of sites that may compliment what Craig has offered you

http://www.gizmag.com/camera-sensor-size-guide/26684/


The second is pretty comprehensive too... this is from one of my favourite UK sites and one I used to post images to. It gets a little technical and gets into diffraction, noise, dynamic range and you can go from there to circles of confusion and resolution There are a few handy calculators on the page too... play with them and you will soon get the idea

http://www.cambridgeincolour.com/tutoria...r-size.htm

All the information is "out there" just the presentation, as far as comprehension is concerned, does vary enormously. The F stop is a function of the lens.... a 100mm f2.8 on a crop is still an f2.8 on a full frame ... I would always buy the fastest lenses I can afford as it make no difference to the lens what camera you put it on. What would change is the effective field of view...

Most wildlife photographers opt for a 1.6 crop sensor...(unless you work for Nat Geo and they give you a top of the range pro full frame and 600 and 800mm lenses!! ) you get away with a lighter, less powerful lens but still get the reach... e.g. a 200mmf2.8 lens on a 1.6 crop sensor will give the equivalent field of view of 320 mm. For that you may have to use a much heavier and expensive say 400mm f2.8 lens. Put then a small lightweight 1.4X converter on your 200mm and on your 1.6 crop frame camera, and you get nearly 500mm EFV! There is a downside though... (always is!) that f2.8 lens is still a 2.8, but due to the light loss through the converter, the equivalent F stop is reduced to probably no more than f 4 or even 5.6 so that restricts shooting time to bright sunlight (stark and not the best) and it may affect the ability of the AF sensor to set focus on some cameras.. or ramp up the ISO/ASA and get progressively more noise that you have to deal with in post later or you have to carry and use a tripod! Photography, my opinion, is very much a balancing act... like high wire walking... get it right and get the plaudits... get it wrong...

Enjoy and I hope it has helped

Kind regards

Rolf

[MrB]

Short answer: No.

Longer answer:

The f number is simply the ratio of the focal length of a lens (F) in mm and the diameter of the aperture (A) in mm = F/A, so its value is independent of the sensor size.

The sensor size affects the field of view - more of the lens's image will fit onto a bigger sensor's rectangle, so it has a wider field of view. When using a 1.5x crop sensor camera and a full-frame sensor camera to get the same field of view (i.e. the same photo), you could either use a 1.5x longer focal length on the full frame camera, or move 1.5x further away from the subject with the crop camera.

In that example, however, if the f number is the same on both cameras, there will be a shorter depth of field (DoF) in the image from the full-frame camera, i.e more out of focus in front of and beyond the subject. But also, in that example, being able to open the aperture by one f-stop on the 1.5x crop camera will give the image a roughly similar depth of field as from the full-frame camera.

So you could think of your investment in fast lenses as beneficial, as it allows you to separate subject from background more effectively and creatively.

Cheers.
Philip

[Rolf]

So you could think of your investment in fast lenses as beneficial, as it allows you to separate subject from background more effectively and creatively.

Cheers.
Philip
[/quote]
Good reply Philip... I wonder if we can have a section on just these kinds of Techy questions that present and future members could access or be pointed to.... the knowledge that you , Ed, Craig and many others have here should be captured and available as a kind of catalogue of FAQ's? Not everyone of course will want to delve so deeply into the science but the simpler stuff of F stops, DoF, Shutter and aperture relationships to light could be just what helps a person to progress especially if they feel a little self conscious when needing to ask.. and may not ask and take up accountancy instead!

Kind regards

Rolf

[JohnBax]

The F Stop just effects the amount of light coming in, and the DoF of course. A Smaller sensor does capture less of that light than a full frame would, howeverit still benefits from the extra light a larger hole lets in.

I wouldn't day it's wasting money to buy fast lenses, as they are still superior to the slower lens you would use instead. And you don't multiply by the same number as you would for focal length.

Tony Northrup has a very detailed video on F Stop and sensor size on YouTube.

Thanks for the prompt replies I do appreciate them. It was Tony Northrup that set me off with this. His theory is that as the focal length changes with the smaller sensors then the F number must change with the calculation that the F no is equal to the focal length divided by the aperture diameter but in my mind the physical length of the lens and the size of the aperture do not change then the F no must be as stated on the lens, hence my question to get other opinions on this point. The other question this raises for me is that if a lens is made for a small sensor i.e. the Sony DT range why is the focal length not as stated as there is no cropping of the image on the sensor?

[Rolf]

With Canon... the "S" series... S standing for Small Image Circle - are designed to match APS-C (blame Kodak and Canon for that!) size sensors. As the image circle to sensor size is relative to that of the standard EF series lenses for full frame sensors, there would be no crop. So, the 10-22mm S series lens is the equivalent view of having a 16-35mm on a full frame... but if you have the 16-36mm on an APS-C 1.6x crop you would be out of the wide angle range with 25.6-56mm at the long end. The "S" series protrudes further back to get the lens closer to the sensor... (this is rubber protected) and would be such that it's ingress into the shutter box of a non APS-C body,would cause the mirror to strike it and jam. To avoid that the mounting reference ("S" series a white square, Ef series a red dot) are different and the "S" series will only mount on bodies with a white reference. The mirror being smaller will not come into contact with the protruding rear. Ef lenses (no protrusions) will mount on both reference bodies. Quoted focal lengths are always the relativity to the "old" film size of 24x36mm - full frame. so... 10mm-22mm is just that on an APS-c sensor but, relative to 24x36 it is 16-35mm exactly the same FoV - just a different way of expressing the same thing .Not sure if that makes sense to you? It was much simpler in days of 35mm film...

Kind regards

Rolf

[JohnBax]

With Canon... the "S" series... S standing for Small Image Circle - are designed to match APS-C (blame Kodak and Canon for that!) size sensors. As the image circle to sensor size is relative to that of the standard EF series lenses for full frame sensors, there would be no crop. So, the 10-22mm S series lens is the equivalent view of having a 16-35mm on a full frame... but if you have the 16-36mm on an APS-C 1.6x crop you would be out of the wide angle range with 25.6-56mm at the long end. The "S" series protrudes further back to get the lens closer to the sensor... (this is rubber protected) and would be such that it's ingress into the shutter box of a non APS-C body,would cause the mirror to strike it and jam. To avoid that the mounting reference ("S" series a white square, Ef series a red dot) are different and the "S" series will only mount on bodies with a white reference. The mirror being smaller will not come into contact with the protruding rear. Ef lenses (no protrusions) will mount on both reference bodies. Quoted focal lengths are always the relativity to the "old" film size of 24x36mm - full frame. so... 10mm-22mm is just that on an APS-c sensor but, relative to 24x36 it is 16-35mm exactly the same FoV - just a different way of expressing the same thing .Not sure if that makes sense to you? It was much simpler in days of 35mm film...

Kind regards

Rolf

Hi Rolf,

Thanks again for your prompt reply. I understand the principles of the relationship between sensor size and focal length and the comparison to 35mm film, but this all started when I watched Tony Northrup's video on the fact that it alters the F no as well as he applies the formula for calculating the f no to the increased length of the lens due to the crop factor and that manufacturers are not giving their customers correct information. In my mind the lens does not increase in length and the aperture stays the same and while there is less light hitting the sensor, the sensor is smaller so the light per sq mm is the same. With regard to the lenses designed for APS C sensor, is the image they project on the sensor the same as a lens for a full size sensor but without the outer portion not projected or does the lens reduce the image to produce the same image as the full frame albeit smaller, if that makes sense.

Regards

John

[Rolf]

[With regard to the lenses designed for APS C sensor, is the image they project on the sensor the same as a lens for a full size sensor but without the outer portion not projected or does the lens reduce the image to produce the same image as the full frame albeit smaller, if that makes sense.

Regards

John

[/quote]

John,
I believe the last point you touched on to be correct in that the light in the "S" series lens construction image circle is all designed to fill the 22x14mm (approx.) sensor. The ef lens would make a much bigger circle to fill a 36x24mm sensor. In both cases the flange to sensor distance is 44 mm, but the last element in the "s" series is closer to the sensor - it protrudes into the shutter box to get closer to the sensor, reducing, proportionally, the image circle to fit I suppose you could liken it to putting smaller wheels on a car... you would have to recalibrate the speedo to account for the smaller diameter and more revolutions per given distance... but the car remains the same and still travels the same distance as on full sized wheels... just does it differently?

This can get quite convoluted

Kind regards

Rolf

[JohnBax]

[With regard to the lenses designed for APS C sensor, is the image they project on the sensor the same as a lens for a full size sensor but without the outer portion not projected or does the lens reduce the image to produce the same image as the full frame albeit smaller, if that makes sense.

Regards

John

John,
I believe the last point you touched on to be correct in that the light in the "S" series lens construction image circle is all designed to fill the 22x14mm (approx.) sensor. The ef lens would make a much bigger circle to fill a 36x24mm sensor. In both cases the flange to sensor distance is 44 mm, but the last element in the "s" series is closer to the sensor - it protrudes into the shutter box to get closer to the sensor, reducing, proportionally, the image circle to fit I suppose you could liken it to putting smaller wheels on a car... you would have to recalibrate the speedo to account for the smaller diameter and more revolutions per given distance... but the car remains the same and still travels the same distance as on full sized wheels... just does it differently?

This can get quite convoluted

Kind regards

Rolf

[/quote]
Hi Rolf,
Thanks for the reply. I know this is all theoretical as the camera takes what it takes but as I am retired now I like to keep the grey matter whirring and this has had me thinking for quite a while now and I am sure it will continue to do so, anyway thanks for your assistance and hopefully I will be a regular contributor to the site if only to ask awkward questions.

Regards

John

[Rolf]

[With regard to the lenses designed for APS C sensor, is the image they project on the sensor the same as a lens for a full size sensor but without the outer portion not projected or does the lens reduce the image to produce the same image as the full frame albeit smaller, if that makes sense.

Regards

John

John,
I believe the last point you touched on to be correct in that the light in the "S" series lens construction image circle is all designed to fill the 22x14mm (approx.) sensor. The ef lens would make a much bigger circle to fill a 36x24mm sensor. In both cases the flange to sensor distance is 44 mm, but the last element in the "s" series is closer to the sensor - it protrudes into the shutter box to get closer to the sensor, reducing, proportionally, the image circle to fit I suppose you could liken it to putting smaller wheels on a car... you would have to recalibrate the speedo to account for the smaller diameter and more revolutions per given distance... but the car remains the same and still travels the same distance as on full sized wheels... just does it differently?

This can get quite convoluted

Kind regards

Rolf

Hi Rolf,
Thanks for the reply. I know this is all theoretical as the camera takes what it takes but as I am retired now I like to keep the grey matter whirring and this has had me thinking for quite a while now and I am sure it will continue to do so, anyway thanks for your assistance and hopefully I will be a regular contributor to the site if only to ask awkward questions.

Regards

John

[/quote]

No problem John, Glad to help in any way I can... as are the other contributors to this forum. I am not that well versed with post production I can do it but as for the finer aspects, EdMak, MrB or English Bob would be better placed to answer those types of questions. I prefer to be involved with the hardware and how stuff works rather than the software side... but in my short time here, it seems we all will pitch in to help another member. Look forward to more of your posts and perhaps even some of your work?

Kind regards

Rolf

[EdMak]

the camera takes what it takes

That's it!!!! the bottom line. Ed.

[Rolf]

that's quite "black n white" Ed...

Kind regards

Rolf

[EnglishBob]

Be sure t watch Northrup's second and more recent video on it, explains further and back tracks a little on his earlier video.

[dave1712]

Hi John,

the crop factor and aperture are 2 different things, although both are related to the focal length of a lens.

When cropped body DSLRs were first developed, most available lenses were for full frame 35mm film SLRs. The concept of crop factor or focal length multipliers was used so photographers could understand what they would get using a full frame lens on a cropped sensor body. Even now, for lenses that can be used on a full frame or cropped body DSLR the focal length is always quoted for full frame.

There are many detailed explanation all over the web on crop factor, and one of the best I've seen is at
http://www.cambridgeincolour.com/tutoria...r-size.htm
in my simplistic explanation: if you put a 100mm lens on a cropped body it is still a 100mm lens and the focusing distance to the sensor is identical to a full frame body - we just capture the centre part of the image area of the lens instead of it all. We effectively reduce the angle of the field of view which has an identical effect to zooming.

The aperture value of a lens is directly related to the diameter of the 'hole' that lets the light through. The diameters are set to either 1/2 or double the area of the hole, letting in 1/2 of twice as much light. To get consistency between lenses the aperture (hole size) is calculated using the focal length. This way a lens at f/2 lens will let the same proportion of light whether its a 100mm of 25mm lens

Note I used f/2 not f2. Over the years for convenience the / has been left out, but it means exactly that: f/2 so for a 100mm lens means diameter of the aperture is 50mm, at f/4 aperture will be 25mm etc (ignoring the high skill of lens and glass designers.) This also explains why f/2 lets in more light than f/8 - for a 100mm lens the aperture is 50mm and 12.5mm diameter respectively)

So say we start with a 100mm lens designed for full frame, which is f/2 (so has a maximum eperture of 50mm diameter.) if we use that on a cropped body camera the aperture is still 50mm diameter, the 100mm lens focal lengh is still the same, as is the distance of the lens from the sensor. The only difference is that with the cropped sensor, we are just using the centre part of the potential image area the lens can produce, which has a zoom effect.

So the simple answer, the crop factor affects the focal length, not the aperture.

Dave

[MrB]

.....in my simplistic explanation: if you put a 100mm lens on a cropped body it is still a 100mm lens and the focusing distance to the sensor is identical to a full frame body - we just capture the centre part of the image area of the lens instead of it all. We effectively reduce the angle of the field of view which has an identical effect to zooming.....

....So say we start with a 100mm lens designed for full frame, which is f/2 (so has a maximum eperture of 50mm diameter.) if we use that on a cropped body camera the aperture is still 50mm diameter, the 100mm lens focal length is still the same, as is the distance of the lens from the sensor. The only difference is that with the cropped sensor, we are just using the centre part of the potential image area the lens can produce, which has a zoom effect.

So the simple answer, the crop factor affects the focal length, not the aperture.

Dave, you have given a good description but then introduced a contradiction in the conclusion (see the bold statements)

The crop factor affects the field of view, as you have written (see the italic statements), not the aperture OR the focal length.

The effect is similar, but not identical, to zooming. If the subject-camera distance and the aperture were kept the same, and the 100mm lens was zoomed to 150mm on the full-frame sensor, you would get the same photo as with the lens at 100mm on a 1.5x crop sensor, because both sensors would get the same field of view. However, the crop sensor image would show a greater depth of field in front of and beyond the subject.

Cheers.
Philip

[dave1712]

Oop| your quite right philip

[JohnBax]

Be sure t watch Northrup's second and more recent video on it, explains further and back tracks a little on his earlier video.

Thanks Bob,
I have watched Tony's films which are up to 4 now, the latest being a rant by a guy called Joe who was thinking on similar lines to me. Having watched this film I understand better what Tony is saying and agree that some manufacturer's do pick and choose how they describe their cameras and it is misleading such as Panasonic stating the camera has a 25-600 lens at f/2.8 which is clearly impossible. I did read somewhere that the aperture is measured by the diameter at the focal plane not the actual opening in the lens which may affect my view if this is the case and to be fair to Tony {possible sign of back pedalling!!} when he takes the photos it does seem correct but he has not tried using different lenses on the same camera i.e. a full frame lens and a cropped lens on an APS C camera. Anyway thanks for taking the time to get involved with my question.

regards

John