Crop sensors and lens focal length effects

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Merkin

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Hello all,

I have never owned a crop sensor digital camera, but I find myself quite tempted to part with the D700 setup I currently have but almost never use for one of the Fuji X cameras. However, never having had to deal with crop sensors before, I have a question about the effect of the sensor size on the general appearance of an image. I am not exactly sure how to phrase it, so I will give an example. Suppose I was using a 28mm lens on a Fuji X-Pro 1, which has a 1.5x crop factor. This makes the lens equivalent to a 42mm normal lens on a 35mm camera. Let us now suppose that I took an up close portrait of someone. Would the portrait turn out to look like a wide-faced slightly bug-eyed portrait like shooting with a 28mm lens on a 35mm camera would, or would it turn out looking like an up close portrait shot with a 42mm lens on a 35mm camera? Conversely, If I were using a 50mm lens (75mm equivalent) with a crop sensor camera, would I get a full frame 50 equivalent level of foreshortening or a full frame 75mm equivalent level of foreshortening? Thanks!
 
If you shoot from the same distance on both cameras then the perspective will be the same. The camera with the smaller sensor will show less area around the subject. If however you adjust the distance to cover the same area, then the 1.5 X sensor camera will have to be half again as far from the subject and the perspective will be flatter.

Clear as mud?
 
Come to think of it, my 4:3 sensor is just about 2X so the math is easy.

If I want to shoot a head and shoulders portrait with my OM-1 and a 50mm lens and have a field of 2X3 feet at the subject plane then I would be about 4ft 6in. from the subject. If I wish to cover the same area (forget the different aspect ratio for a moment) with the same 50mm lens mounted on my E-410, then I would have to stand 9 feet from the subject and the perspective would change, it would be more flat.
 
zuiko85 said:
If you shoot from the same distance on both cameras then the perspective will be the same. The camera with the smaller sensor will show less area around the subject. If however you adjust the distance to cover the same area, then the 1.5 X sensor camera will have to be half again as far from the subject and the perspective will be flatter.

Clear as mud?
Click to expand...

Let me see if I get this. Let's assume that I have two cameras, one 35mm film, and one 1.5x crop digital, both with 28mm lenses. Let's assume I am doing a headshot of someone. With the film camera, I am standing 1 meter away. To get the exact same composition with the digital, I would be standing 1.5 meters away. From 1.5 meters away with the digital, the image would look the same as far as distortion goes as a 42mm lens on a 35mm film camera would. Is that it?
 
Merkin, I've had to do this math way before the digital era. I've shot with Olympus OM full frame and Pen F half frame cameras side by side for 40 years. The factor for half frame is about 1.4X.
 
Merkin, I think you got it! Just remember;
Only the distance from the subject controls perspective, regardless of focal length.
 
Cool cool, many thanks! I shoot with a Pen FT, but as I just use it as a street photography camera, I've never really paid much attention to it. I tend to use medium format for proper film portraiture. However, most of the time I end up dragging out the D700, I am doing portraiture, so I needed to make sure that I would be ok using a crop sensor with shorter focal lengths. I really appreciate it.
 
Merkin, I feel for you as it took me a while to get my head around it too.

This is how I find it...
If I shoot my 21SA on my R-D1 (1.5 crop) and my other 21SA on my M2 from the same distance, it just looks like I took a pair of scissors to the R-D1 photo and snipped off the top, bottom and sides a bit. 'Distortion' in both photos remains the same.

If I want the R-D1 image to match the area of the M2 image I have to step back half as far from the spot that I'm shooting the M2.

If I shoot the 21SA on the R-D1(1.5 crop) side by side with my 35 summilux on the M2, the images will cover roughly the same in area but the R-D1 image will still retain the wide angle 'look', whereas the 35 summilux will be what you expect from a 35.

Hope this clears it up a little...:)

added:

Just got myself a pen-ft too! Great little camera. There's also a fair few 21SA pics shot on the R-D1 in my gallery if you want to see examples.
 
The focal length doesn't get "cropped," the lens's angle of view gets cropped. So that there are no "focal length effects," only angle of view effects.

I swear, one of these days I'm going to post a chart of focal lengths and their horizontal angles of view, so that we can stop talking in this imprecise terminology and instead refer directly to angle of view.

Myself, I love that 62 degree angle of view. ;)

-Joe
 
Joe, I didn't want to introduce angle of view also, at least not just yet. Just the practical aspects of handling various lenses on various size sensors. Most of us, when composing a shot, know how much we want to include (or exclude ) from the frame. We then move backward or forward with a fixed focal length or perhaps change lenses or adjust focal length if we have a zoom lens mounted. Us old timers, and I presume folks with digital also crop a frame when we could not or did not get it right with the initial shot. I think cropping is a lot harder with a negative in the darkroom than with a digital file in an computer. I don't know from computers and still wet print and so I try to get it right in frame. You sure don't want to crop a 18X24mm negative! It's small enough already.
 
I guess what the OP is asking is if the lens distortion from a wide-angle is negated by the crop-factor/distance from the subject.
 
JoeV said:
I swear, one of these days I'm going to post a chart of focal lengths and their horizontal angles of view, so that we can stop talking in this imprecise terminology and instead refer directly to angle of view.
Click to expand...

Yes but it is easier to say XXmm lens equivalent or equivalent to a XXmm on a 35mm/FF camera, than it is to say XXmm lens equivalent angle of view, or equivalent angle of view for XXmm lens on a 35mm/FF camera. Those who know it doesn't mean focal length adopt a less wordy way to make a statement, but is understood. Unfortunately they take the easy way out, instead of being more precise (often more wordy) for the benefit of those who are just learning.
 
uinku said:
I guess what the OP is asking is if the lens distortion from a wide-angle is negated by the crop-factor/distance from the subject.
Click to expand...

If the lens has barrel or pincushion distortion (most likely barrel for a 28mm lens designed for 24x36mm format), then that'll be there no matter what part of the image you use, however that kind of distortion is generally worse at the edge, so using a smaller sensor will help.

There's also the matter of subject distance. Let's take the example of a 21mm lens for 24x36 used for a tight head shot. Assuming the lens can even focus that close, you'll be right on top of your subject (about 10"/25cm away or so). The nose will look much larger than the ears because it's so much closer to the camera, the ears are possibly as much as 1.5x the distance to the camera than the nose is. Using a 210mm lens puts you 10x further away for the same image, so the difference in distance between the nose and ears is much less, so they'll appear more normal, maybe even a bit flattened due to the "telephoto" effect.

When people talk about wide angle lenses distorting, it's not necessarily barrel distortion, but just the effect of close and far objects being exaggerated due to the angle of view and working distance.

That's a long way to say that if you use a 28mm lens on 16x24mm to get the same image as a 42mm lens on 24x36mm format, you'll have to be about the same distance, so ideally the images will look nearly identical, assuming neither lens has any strong optical distortion (barrel or pincushion) due to the design.
 
Merkin, people make this more complicated than it is ....

Say you put your camera on a tripod, and you don't move it:

If you now change focal length, for example from 25mm to 50mm, the resulting picture will be identical as if you would crop the original picture half on either side.

The same will happen if you move from a full frame sensor to using a crop sensor of factor 2.

The perspective distortion that you asked for will change much like when you change focal length on a film camera.

Roland.
 
astro8 said:
Merkin, I feel for you as it took me a while to get my head around it too.

This is how I find it...
If I shoot my 21SA on my R-D1 (1.5 crop) and my other 21SA on my M2 from the same distance, it just looks like I took a pair of scissors to the R-D1 photo and snipped off the top, bottom and sides a bit. 'Distortion' in both photos remains the same.

If I want the R-D1 image to match the area of the M2 image I have to step back half as far from the spot that I'm shooting the M2.
Click to expand...

Beautifully explained. In other words, the perspective does not change.
 
Merkin,
Astro8 described it very well. The operative word here is "crop". Using a smaller sensor just gives you a crop of exactly the same picture you would have with full-frame sensor, just as if you would crop the edges in the darkroom or in post processing. Everything else remains the same (depth, distortion, etc.).
Multiplying the "crop factor" with the wavelength only gives you an idea of the field angle when compared to full frame.
 
zuiko85 said:
Enough of this chit chat. Lets talk about something easy.....
like depth of field.:rolleyes:
Click to expand...
1st photographer: Needs more depth of field. :angel:

2nd photographer: Needs less depth of field. :mad:

3rd photographer: Bokeh is overrated. :rolleyes:

4th photographer: You guys need to step away from the computer and make some photos. :D

That about covers it.
 
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