Crop Sensor Format / Subject Distance / Gauging an equivalent full-frame AOV

character_stops
Enthusiast

While using the crop sensor format, how does one adjust the subjects distance to have the same field of view (AOV) as a full-frame counterpart, assuming the current focal length in use has already been gauged to reflect the 35-mm film standard? I have the EOS Rebel SL1.

3 ACCEPTED SOLUTIONS


@character_stops wrote:

Yes. I'm pretty sure the camera I have is smaller format. That's why I think I should experiment with more lenses to find out how each one responds. I think the format for my camera is 2x3 but I'm not completely sure.  After putting in my cameras largest image size in Ps, I selected a photo preset, and it converted it to 2x3 inches. Would that mean a crop factor much more than 7.5?


Don't confuse "cropping" in Photoshop with "lens/sensor crop factors" with respect to your camera.

 

Photoshop (and many other image softwares) have a number of preset common image aspectg ratios for cropping purposes. Choosing 2:3 in Photoshop will render the entire image area that's produced by your camera. You can then resize to a number of common paper sizes.... 4x6 inches, 6x9 inches, 8x12 inches, 12x18 (or 13x19) inches... that all share the same image aspect ratio.

 

If you instead choose 4:5 aspect ratio in the software cropping tool, some of the longer axis of the image will be trimmed off, but you can then resize the image to fit other common paper sizes such as 4x5 inches, 8x10 inches, 16x20 inches.

 

There are also some common paper sizes that don't exactly conform to any of the preset aspect ratios, such as  5x7 inches and 11x14 inches. These are somewhere in-between.

 

Your camera has a 2:3 (not 2x3) aspect ratio. This has been fairly standard among 35mm film SLRs and has been carried over to digital, too (for the large part... there are a few exceptions). The actual size of the sensor in your camera is approx. 15x22mm. So-called "full frame" cameras also have a 2:3  aspect ratio... the actual size of their sensor is approx. 24x36mm.

 

In other words, the same aspect ratio (i.e "short side vs long side") is possible in different sizes (both in the sensor or film format... and in printing paper sizes). Don't worry too much about reference to large format... That appeared to have been mentioned just to demonstrate that there are all sorts of "lens factors" that you can get caught up in calculating, but it really doesn't solve much.

 

Lens focal length doesn't change. 50mm is 50mm no matter what.

 

But a 50mm lens on one format will behave differently than it does on another format. On your camera with it's 15x22mm sensor a lens with 50mm focal lenghts acts as a nice "short telephoto".  Put the exact same focal length on a point-n-shoot digital which has a really small sensor and it can behave as a very powerful, long telephoto.  Or, put that same 50mm on a "full frame" digital camera and it will act as a "standard" lens (i.e., not wide or telephoto). Or, put that same 50mm on a medium format film camera that has a 60x70mm image area and it will be a wide angle lens. Finally, put it on a large format film camera such as a 4x5 (inches... equal to approx 100x125mm) and the same 50mm lens focal length behaves as an extreme wide angle.

 

You say you have a kit lens.... I would assume it's a zoom. Probably an EF-S 18-55mm or an EF-S 18-135mm. You also have an EF 50mm "prime" (not a zoom) lens. If you set the EF-S zoom to 50mm it will give exactly the same angle of view as the EF 50mm prime lens. The only difference is that the EF-S lens is designed specifically for smaller sensor cameras, while the EF lens is designed to give a large enough image circle to cover the area of a full frame sensoe camera.

 

Yes, it's true, because they don't need to produce as large an image circle EF-S or "crop only" lenses made for use on smaller sensor cameras can be smaller in size, lighter weight and less expensive than lenses made to cover a full frame sensor's image area.

 

If your kit lens is an 18-55mm, on your camera that provides a variable angle of view ranging from moderate wide angle to standard to short telephoto. If it's an 18-135mm kit lens, that gives a little more tele range, to a moderate telephoto.  

 

I hope this makes sense!

View solution in original post

ebiggs1
Legend
Legend

It "is simply the ratio of the diagonal ..."  Which is much easier to understand.

I actually have no idea what the OP is trying to say?   Smiley Frustrated

 

The angle of acceptence is the same if the focal length is the same.  It makes no difference what camera it is on.  The coverage that angle produces it what is different.  The subject distance would have to be adjusted accordingly.

A lens can not change. If it is a 50mm lens, it is 50mm on whatever camera you might put it on.

 

Every lens has its Circle of Coverage.  The lens projects a circular image on the sensor.  This is what is different from a FF (35mm format) vs an APS-C camera.

EB
EOS 1DX and 1D Mk IV and less lenses then before!

View solution in original post

ebiggs1
Legend
Legend

"If you set the EF-S zoom to 50mm it will give exactly the same angle of view as the EF 50mm prime lens. The only difference is that the EF-S lens is designed specifically for smaller sensor cameras, while the EF lens is designed to give a large enough image circle to cover the area of a full frame sensoe camera."

 

This is a little confusing.  It is basicly correct, I think?  Comparing a 50mm prime and a zoom that is set to 50mm, they are identical.  It makes no differenc that one is a EF and the other is a EF-S lens.  Focal length is focal length, is focal length, it matters not what its intended use was/is.

EB
EOS 1DX and 1D Mk IV and less lenses then before!

View solution in original post

8 REPLIES 8

amfoto1
Authority

@character_stops wrote:

While using the crop sensor format, how does one adjust the subjects distance to have the same field of view (AOV) as a full-frame counterpart, assuming the current focal length in use has already been gauged to reflect the 35-mm film standard? I have the EOS Rebel SL1.


First, have you used 35mm film cameras and lenses a lot?

 

If so, you could multiply or divide everything by 1.6X.

 

I just estimate based upon experience. For example, I know a 28mm on an APS-C camera gives me similar AOV to the standard lens I used on my old film cameras... 10 or 12mm is ultrawide.... 200mm acts pretty close to the way a 300mm lens did on my film cameras... 300mm "acts like" a 500mm lens did on the old cameras.My 70-200mm lenses behave like 112-280mm on film. Etc.

 

If you haven't ever used 35mm cameras, you shouldn't have any preconceived notions and don't really need to think about it too hard. Focal lengths don't actually change. Only the way those focal lengths "behave" actually changes with different sensor size.

 

On an APS-C camera like your SL1...

 

10 to 15mm is ultrawide

16 to 24mm is wide to slightly wide angle

25 to 35mm is standard

40 to 60mm is very short telephoto

75, 85 and 100mm is short telephoto

135, 150 and 200 are moderate telephoto

250mm and up are super telephoto

 

Hope this helps!

 

***********


Alan Myers
San Jose, Calif., USA
"Walk softly and carry a big lens."
GEAR: 5DII, 7D(x2), 50D(x3), some other cameras, various lenses & accessories
FLICKR & EXPOSUREMANAGER 

Well, actually I haven't. I have the EF-S kit lens and the ES 50-mm prime lens, which is capable of shooting at 35-mm. I have an article though that point out how crop sensors allow lenses to be much smaller than they would need to be or to achieve equivalent focal lengths that are desired.This made me wonder how that helps if I wanted to get the widest angle out of the two lenses I've got. They're both suited for crop sensor format.

 

I like how you've committed to memory similar angles paired with the different lenses in both formats and that the focal lengths stay fixed. In my own research, if one uses the same lens on either format with the same F-NUMBER, the crop sensor will yield a smaller AOV compared to the full-frame DOF, which would be inverse in proportion and have a shallower focus.

 

I think what's got me confused is another aspect from the same article involving DSLR wide-angle lenses, therein the former AOV is EQUIVALENT to the LONGER FOCAL LENGTH of the latter. It says while using the SAME LENS on either format you can adjust the subject distance to get the same AOV. Moreover, by saying 'adjust the subject distance', I think they simply mean to frame the subject the same way though the viewfinder.

 

I guess that explains that one's only choice if they desire a larger AOV would be to use whichever lens has the shortest focal length and shoot with the same F-NUMBER they know works with the FULL FRAME counterpart they have experience with, and which has previously given them their desired DEPTH as well. Isn't one a reciprocal of the other as long as one uses the same F-NUMBER?

ebiggs1
Legend
Legend

"If you haven't ever used 35mm cameras, you shouldn't have any preconceived notions and don't really need to think about it ..."

 

Not only is this so very true, you shouldn't think about it at all anyway.  You need to know how a specific lens responds on your camera for its own self.  You know medium and large format camera users don't try to convert their lenses to FF specs!  Crop factor is simply the ratio of the diagonal lengths of two (35mm vs APS-C) formats, or 1.6.  

 

BTW, if you are using a 8x10 large format camera the crop factor is approx. 7 1/2.

 

EB
EOS 1DX and 1D Mk IV and less lenses then before!

Yes. I'm pretty sure the camera I have is smaller format. That's why I think I should experiment with more lenses to find out how each one responds. I think the format for my camera is 2x3 but I'm not completely sure.  After putting in my cameras largest image size in Ps, I selected a photo preset, and it converted it to 2x3 inches. Would that mean a crop factor much more than 7.5?


@character_stops wrote:

Yes. I'm pretty sure the camera I have is smaller format. That's why I think I should experiment with more lenses to find out how each one responds. I think the format for my camera is 2x3 but I'm not completely sure.  After putting in my cameras largest image size in Ps, I selected a photo preset, and it converted it to 2x3 inches. Would that mean a crop factor much more than 7.5?


2x3 is the aspect ratio, i.e. the ratio of the height to the width of the sensor. The sensors of most modern DSLRs use that aspect ratio. It's a dimensionless number and says nothing about the size of the sensor or of any image made from it. Your photo editor arbitrarily picked 2x3 inches as the size to display the image.

 

The "crop factor" of 1.6 is something entirely different. It's the ratio of the surface area of a full-frame sensor (864 sq mm, IIRC) to that of the sensor in a Canon crop-frame DSLR.

Bob
Philadelphia, Pennsylvania USA


@character_stops wrote:

Yes. I'm pretty sure the camera I have is smaller format. That's why I think I should experiment with more lenses to find out how each one responds. I think the format for my camera is 2x3 but I'm not completely sure.  After putting in my cameras largest image size in Ps, I selected a photo preset, and it converted it to 2x3 inches. Would that mean a crop factor much more than 7.5?


Don't confuse "cropping" in Photoshop with "lens/sensor crop factors" with respect to your camera.

 

Photoshop (and many other image softwares) have a number of preset common image aspectg ratios for cropping purposes. Choosing 2:3 in Photoshop will render the entire image area that's produced by your camera. You can then resize to a number of common paper sizes.... 4x6 inches, 6x9 inches, 8x12 inches, 12x18 (or 13x19) inches... that all share the same image aspect ratio.

 

If you instead choose 4:5 aspect ratio in the software cropping tool, some of the longer axis of the image will be trimmed off, but you can then resize the image to fit other common paper sizes such as 4x5 inches, 8x10 inches, 16x20 inches.

 

There are also some common paper sizes that don't exactly conform to any of the preset aspect ratios, such as  5x7 inches and 11x14 inches. These are somewhere in-between.

 

Your camera has a 2:3 (not 2x3) aspect ratio. This has been fairly standard among 35mm film SLRs and has been carried over to digital, too (for the large part... there are a few exceptions). The actual size of the sensor in your camera is approx. 15x22mm. So-called "full frame" cameras also have a 2:3  aspect ratio... the actual size of their sensor is approx. 24x36mm.

 

In other words, the same aspect ratio (i.e "short side vs long side") is possible in different sizes (both in the sensor or film format... and in printing paper sizes). Don't worry too much about reference to large format... That appeared to have been mentioned just to demonstrate that there are all sorts of "lens factors" that you can get caught up in calculating, but it really doesn't solve much.

 

Lens focal length doesn't change. 50mm is 50mm no matter what.

 

But a 50mm lens on one format will behave differently than it does on another format. On your camera with it's 15x22mm sensor a lens with 50mm focal lenghts acts as a nice "short telephoto".  Put the exact same focal length on a point-n-shoot digital which has a really small sensor and it can behave as a very powerful, long telephoto.  Or, put that same 50mm on a "full frame" digital camera and it will act as a "standard" lens (i.e., not wide or telephoto). Or, put that same 50mm on a medium format film camera that has a 60x70mm image area and it will be a wide angle lens. Finally, put it on a large format film camera such as a 4x5 (inches... equal to approx 100x125mm) and the same 50mm lens focal length behaves as an extreme wide angle.

 

You say you have a kit lens.... I would assume it's a zoom. Probably an EF-S 18-55mm or an EF-S 18-135mm. You also have an EF 50mm "prime" (not a zoom) lens. If you set the EF-S zoom to 50mm it will give exactly the same angle of view as the EF 50mm prime lens. The only difference is that the EF-S lens is designed specifically for smaller sensor cameras, while the EF lens is designed to give a large enough image circle to cover the area of a full frame sensoe camera.

 

Yes, it's true, because they don't need to produce as large an image circle EF-S or "crop only" lenses made for use on smaller sensor cameras can be smaller in size, lighter weight and less expensive than lenses made to cover a full frame sensor's image area.

 

If your kit lens is an 18-55mm, on your camera that provides a variable angle of view ranging from moderate wide angle to standard to short telephoto. If it's an 18-135mm kit lens, that gives a little more tele range, to a moderate telephoto.  

 

I hope this makes sense!

ebiggs1
Legend
Legend

It "is simply the ratio of the diagonal ..."  Which is much easier to understand.

I actually have no idea what the OP is trying to say?   Smiley Frustrated

 

The angle of acceptence is the same if the focal length is the same.  It makes no difference what camera it is on.  The coverage that angle produces it what is different.  The subject distance would have to be adjusted accordingly.

A lens can not change. If it is a 50mm lens, it is 50mm on whatever camera you might put it on.

 

Every lens has its Circle of Coverage.  The lens projects a circular image on the sensor.  This is what is different from a FF (35mm format) vs an APS-C camera.

EB
EOS 1DX and 1D Mk IV and less lenses then before!

ebiggs1
Legend
Legend

"If you set the EF-S zoom to 50mm it will give exactly the same angle of view as the EF 50mm prime lens. The only difference is that the EF-S lens is designed specifically for smaller sensor cameras, while the EF lens is designed to give a large enough image circle to cover the area of a full frame sensoe camera."

 

This is a little confusing.  It is basicly correct, I think?  Comparing a 50mm prime and a zoom that is set to 50mm, they are identical.  It makes no differenc that one is a EF and the other is a EF-S lens.  Focal length is focal length, is focal length, it matters not what its intended use was/is.

EB
EOS 1DX and 1D Mk IV and less lenses then before!
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