Understanding Sensor Cropping and Lens Conversion

RobCrist1988

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Hello friends!

I've been reading around the interwebs and I'd like to validate what I think is correct about using my 600D's 1.6x cropped sensor and matching it with equivalent 35mm lenses.

To my understanding I need to step every lens I use by a factor of 1.6x - so an 80mm lens on my 600D would be the equivalent of a 50mm lens on a full frame sensor, such as the 5D. Likewise, a 320mm lens on my 600D would be the same as a 200mm lens on a 5D.

EDIT: Actually, in thinking about it, to get as close to 50mm as possible I'd have to step down to a 35mm... I think...

Am I understanding this correctly or do I have it reversed?

The reason for my asking is I'm looking at different lens choices as I learn how to use my camera. For indoor portrait shots I understand that a 50mm lens is ideal for indoor rooms where you want to capture good bokeh. If that's what I'm wanting to eventually get, I'd need an 80mm lens for my camera if I'm understanding this conversion correctly.

There's also the remote possibility that it doesn't make THAT much of a difference, but if photography is just as much a science as it is an art, I'd rather be accurate :)
 
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There's also the remote possibility that it doesn't make THAT much of a difference

This is partly true. Back in the days when film was ruling the camera world the 35mm film camera was THE top choice. Especially of the hobby/amateur market, but it was also widely used in the professional market as well. When digital cameras first started to come out the cost and complexity of producing large silicon wafers for the sensors meant that many companies didn't make 35mm equivalent sizes. As a result when people started changing from film to digital the "crop factor" became part of the process of changing because many of the new digital cameras were "cropped" sensors compared to the 35m film they were used to.

35mm just became a standard measuring point - even though in the film days there were many different sizes of film back then. Today if you're starting fresh and don't have any prior 35mm film/sensor experience the crop factor is partly meaningless because you've no experience of the lens on a different sized sensor so you've nothing to relate it too.

You will want to look up perspective distortion though; for indoor portraits and similar photography using a 50mm lens or longer is often the choice of many photographers so that they can shoot from a good distance away from the subject for a portrait and not have perspective distortion affect the final shot. With crop sensors people tend to end up reaching for a 35mm or similar lenses which can increase the chances of perspective distortion (as you're moving into wide angle lenses and away from telephotos).

Perspective distortion is where areas of the subject closer to the camera are enlarged over those further back - big noses, lips, hands etc...
 
Ok.. well it has to do with your angle of view. On a crop sensor that has a 1.6 crop factor you are using less area of the sensor, so your photo is being cropped for you by 1.6 times. This makes it appear like the elements in your image are actually 1.6 times closer to you than they really are, but it is not actually changing the focal length of the lens, it is changing your angle of view.

So if you have a 50 mm lens and your using a full frame camera, then the picture will appear to have been taken at 50 mm when you compare it to the same picture taken by a 35 mm camera also at 50 mm. If you take the same picture with a 50 mm lens using a camera with a 1.6 crop factor then the picture will appear as if it were taken by an 80 mm lens instead, because on the crop sensor camera the camera is actually cropping the photo for you and changing your angle of view.

A 50 mm prime on a full frame camera would probably be fine for indoor use, whereas for smaller rooms a 50 mm on a crop sensor camera will most likely be a little too much zoom in many situations.
 
It all comes down to angles of view and ratios. A 35mm film frame is 36mm x 24mm. Divide the lens focal length by the frame size (50mm / 36mm = 1.388...) and use that ratio to find the equivalent focal length for any sensor size. A Canon APS-C frame is 22.2mm x 14.8mm (1.6x crop factor), so the equivalent focal length is 1.388... times 22.2mm, which is 30.8mm. That is, a 30.8mm lens on a "crop" sensor has the same angle of view as a 50mm lens on a 35mm or "full frame" digital camera.

As another example, let's take a small sensor, 1/1.7" (7.6mm x 5.7mm). The equivalent focal length would be 1.388... times 7.6mm, which is 10.6mm. A lot of point-and-shoot cameras (which typically have small sensors) have actual focal lengths in that range.

Note that, for simplicity, I'm using only the long dimension of each format, and not all formats have the same aspect ratio (some are closer to square than others). It may be better to compare sensors by their diagonal measurement instead. The same principles apply if you want to do that, but be sure to use the same measurement for all formats.
 
As mentioned above, crop sensors 'see' only the central portion of the image produced out the back end of a lens. If a full-frame EF lens is mounted on your 600D, the 'dinner plate size' image would only have a 'pie plate size' sensor to get the picture. The outer edges of all 4 sides of the frame would 'lost'...eg...'cropped'. I tend to think of it more like putting horse-blinders on the lens preventing it from 'seeing' as far to the right/left/top/bottom as a full frame camera sensor would with the exact same lens mounted. So, mounting a lens with a focal length of 50mm on your 600D would result in a theoretical field of view of an 80mm lens (50x1.6=80). Using the Canon lens comparison table http://www.usa.canon.com/app/pdf/lens/EFLensChart.pdf for a 50mm f1.8 ii lens, the horizontal field of view is 40 degrees, when mounted on a full frame (35mm film size) camera, whereas an 85mm f1.8 lens (closest to theoretical 80mm) is 24 degrees...quite a bit narrower. So, to get the same image as that 50mm lens on a full frame would 'see', you'll have to back up 5-10 feet or so to see the same thing in your viewfinder.

On the other hand, forums such as this one and others make way too much about cropped sensors as being 'inadequate' or 'not good enough', etc. In my book, that's complete BALONEY! I shot 35mm for a good 30-35 years before moving to a point and shoot, and then a crop-sensor DSLR and was completely ignorant of the 'fact' that my camera was 'crippled' or 'limited' in any way. When I saw the image I wanted to capture in the viewfinder, I took the picture. I was perfectly happy with the comparatively 'super-cropped' sensor point and shoot Canon G3 and G5.

In short, in my mind, trying to keep doing mental gymnastics of 'well this lens on my camera is... vs if I spend an extra $1000 or more and get a full frame camera this lens is ...' is a complete waste of time. Regardless of the camera, regardless of the lens, if you see what you want to photograph in the viewfinder, take the picture!
 
To my understanding I need to step every lens I use by a factor of 1.6x - so an 80mm lens on my 600D would be the equivalent of a 50mm lens on a full frame sensor, such as the 5D. Likewise, a 320mm lens on my 600D would be the same as a 200mm lens on a 5D.


I think that it is important to point out that you have it backwards here. It doesn't work the same way going from a cropped sensor to a full frame as it does going from a full frame to a crop sensor. So, if you put an 80mm lens on your 600D, then its going to be the equivalent of 130.
 
Today if you're starting fresh and don't have any prior 35mm film/sensor experience the crop factor is partly meaningless because you've no experience of the lens on a different sized sensor so you've nothing to relate it too.

Bingo. Seems as if a lot of folks get too caught up in the sensor size argument but don't really know what it means. Many things are kept from the film days that don't really apply to digital.
 
To my understanding I need to step every lens I use by a factor of 1.6x - so an 80mm lens on my 600D would be the equivalent of a 50mm lens on a full frame sensor, such as the 5D. Likewise, a 320mm lens on my 600D would be the same as a 200mm lens on a 5D.


I think that it is important to point out that you have it backwards here. It doesn't work the same way going from a cropped sensor to a full frame as it does going from a full frame to a crop sensor. So, if you put an 80mm lens on your 600D, then its going to be the equivalent of 130.


yeah, the more you crop a sensor, the more you "zoom" in.
 
"C'mon, fellas, it's all ball bearings these days." - Chevy Chase in "Fletch"
 
Hello again, everyone. Thank you for clearing up the issue of cropped sensor sizes and lens conversions. I appreciate all the feedback!

In short, in my mind, trying to keep doing mental gymnastics of 'well this lens on my camera is... vs if I spend an extra $1000 or more and get a full frame camera this lens is ...' is a complete waste of time. Regardless of the camera, regardless of the lens, if you see what you want to photograph in the viewfinder, take the picture!

Bingo. Seems as if a lot of folks get too caught up in the sensor size argument but don't really know what it means. Many things are kept from the film days that don't really apply to digital.

I absolutely agree with this statement here. I've been taking a lot of shots the last few days (which is why it's taken me a while to reply) and with absolutely no 35mm experience I don't see what all the fuss is about unless you're use to using 35mm or full-frame setups and have lenses that you expect to perform in certain ways from that experience. On the other hand, if you're doing any kind of telephoto photography, I can see an advantage to having a cropped sensor with a similar lens for zoom purposes. I don't think cropped sensors have a 'worse' quality of photo than a full-frame sensor, though mechanically I can see where full-frame sensors would fair better in low-light.


I think that it is important to point out that you have it backwards here. It doesn't work the same way going from a cropped sensor to a full frame as it does going from a full frame to a crop sensor. So, if you put an 80mm lens on your 600D, then its going to be the equivalent of 130.

Absolutely. Now it's clear to me that if I want to use a lens that would produce a certain 'zoom' at 50mm on a full-frame, I'd need to divide 50 by 1.6 (in my case the 600D has a 1.6 cropped sensor) and be sitting at 31.25mm, or 32mm.


About the only other consideration I feel I'll need to eventually make stems from when I become far more serious about my photography than I am now in the learning stages. Eventually I can see myself wanting to pick up a 5D Mark III or other some such full-frame camera. If I do, while I can still use the lenses I'm using now and will pick up as I move forward, I'll end up having to get a bunch of new lenses for my future 5D to produce similar photos with the same composition. Other than that, sensor size doesn't seem to be a thing at this point.
 
Hello again, everyone. Thank you for clearing up the issue of cropped sensor sizes and lens conversions. I appreciate all the feedback!

In short, in my mind, trying to keep doing mental gymnastics of 'well this lens on my camera is... vs if I spend an extra $1000 or more and get a full frame camera this lens is ...' is a complete waste of time. Regardless of the camera, regardless of the lens, if you see what you want to photograph in the viewfinder, take the picture!

Bingo. Seems as if a lot of folks get too caught up in the sensor size argument but don't really know what it means. Many things are kept from the film days that don't really apply to digital.

I absolutely agree with this statement here. I've been taking a lot of shots the last few days (which is why it's taken me a while to reply) and with absolutely no 35mm experience I don't see what all the fuss is about unless you're use to using 35mm or full-frame setups and have lenses that you expect to perform in certain ways from that experience. On the other hand, if you're doing any kind of telephoto photography, I can see an advantage to having a cropped sensor with a similar lens for zoom purposes. I don't think cropped sensors have a 'worse' quality of photo than a full-frame sensor, though mechanically I can see where full-frame sensors would fair better in low-light.


I think that it is important to point out that you have it backwards here. It doesn't work the same way going from a cropped sensor to a full frame as it does going from a full frame to a crop sensor. So, if you put an 80mm lens on your 600D, then its going to be the equivalent of 130.

Absolutely. Now it's clear to me that if I want to use a lens that would produce a certain 'zoom' at 50mm on a full-frame, I'd need to divide 50 by 1.6 (in my case the 600D has a 1.6 cropped sensor) and be sitting at 31.25mm, or 32mm.

About the only other consideration I feel I'll need to eventually make stems from when I become far more serious about my photography than I am now in the learning stages. Eventually I can see myself wanting to pick up a 5D Mark III or other some such full-frame camera. If I do, while I can still use the lenses I'm using now and will pick up as I move forward, I'll end up having to get a bunch of new lenses for my future 5D to produce similar photos with the same composition. Other than that, sensor size doesn't seem to be a thing at this point.


Well really the only "fuss" involved would be in a situation where you are trying to decide on what lens to purchase you just need to keep in mind the situations your using it in and how much room you have to work. If your lens is 50 mm and your working with a full frame camera it means you won't have to back up quite as far to get everything in the frame. Put the exact same lens on a crop sensor and the equation changes somewhat. Outdoors where you have room to back up, it's not an issue at all. Indoors however, when your working in small rooms or confined spaces, then it might become an issue.
 
Correct. If you're using a 50mm now, and upgrade to a FF sensor, putting that same 50mm on the FF will frame more like if as you've been shooting a 35mm on your current body.


The easiest thing to do is simily not care about the FF equivalent until it's nessecary to care (planning an upgrade).
 
The main reason that I was concerned about pointing out the diffidence I did is that ther are lenses that are designed for cropped sensors, and I am worried you'll apply your logic to them. For example, let's say you have a 50 on a full frame camera, and put it on a cropped sensor. You'll get an image with the field of view of a roughly 80mm lens. If you take a 50 designed for a cropped sensor, your going to get an image about the size of a cropped sensor surrounded by a black circle.
 
The main reason that I was concerned about pointing out the diffidence I did is that ther are lenses that are designed for cropped sensors, and I am worried you'll apply your logic to them. For example, let's say you have a 50 on a full frame camera, and put it on a cropped sensor. You'll get an image with the field of view of a roughly 80mm lens. If you take a 50 designed for a cropped sensor, your going to get an image about the size of a cropped sensor surrounded by a black circle.

Correct. The fact that a lens is designed for crop sensor (like a Nikon DX) doesn't change the equation because it's still being compared to the same lens used on a full frame or the old 35 mm format. The DX lenses for Nikon or other lenses that are designed specifically for crop sensors just save you a little money vrs buying the same lens designed to work for a full frame (FX in Nikon terms). Since the lens only has to project to a smaller portion of the sensor it means they can use less glass, fewer materials, etc.

The upside is they are cheaper. The downside is if you ever do decide to switch to full frame you won't be able to use the lenses designed for crop sensors and you'll need to get FX lenses to replace them. But a DX lens at 50 mm on a crop sensor body will still give you the same angle of view you would get from an 85 mm lens on a full frame or a 35 mm camera.

Hope that makes sense :)
 

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