Lens characteristics different between full frame and APS sized frame.

Hello,

I have just purchased my first DSLR body (Canon 20D) and I'm looking for lenses. One downside of the 20D is of course the APS sized sensor. I would like to know how lenses will differ between use on full frame and APS frame.

I understand that the image will be cropped, which makes sense as the sensor is smaller. But I've also read that the DOF will change, if true why is this? I can't really understand the logic behind that.

And my second question: are there any other characteristics that will change when used on APS sized sensor cameras?

Thanks for your help!

The APS crop has influence on the angle of view. Let's say you just made a picture with a FF camera. To get identical framing on APS, you have to get further from the subject. This is the reason for a shallower DOF on a FF camera.

Tamron has a nice interactive app on their website you might want to look at.

http://www.tamron.com/lenses/learning_center/tools/depth-of-field-comparison.php

Hans

--
Digital pictures are born analog. Hence, analog rules.

Hans,
Thanks for your reply, this makes a lot of sense to me!

So the lens characteristic doesn't change, but you have to use it in a different way because of the crop with different results as a consequence.

So using a lens on APS frame results either in: 1. cropped frames or 2. shallower DOF. Do I understand correctly?

Thanks again!

2manydjs said:

Hans,
Thanks for your reply, this makes a lot of sense to me!
So the lens characteristic doesn't change, but you have to use it in
a different way because of the crop with different results as a
consequence.

So using a lens on APS frame results either in: 1. cropped frames or
2. shallower DOF. Do I understand correctly?

Thanks again!

--

Click to expand...

You're welcome. A lens is a lens, lens characteristics don't change.

1. is correct.
2. is valid on a FF camera.

Hans

--
Digital pictures are born analog. Hence, analog rules.

The only practical difference is in the cropping effect which means that some FF lenses do not have such a useful effective focal length range on a crop camera.

E.g: a 50mm prime is a very useful general purpose prime lens on FF but on a DX crop camera it is really a head and shoulders portrait lens. An equivalent prime lens on a DX would be around 30mm.

Another example is the Canon 28-135 IS lens which is a very good general purpose zoom on FF but isn't really wide enough on a DX. However, the Canon EF-S 17-85mm IS has exactly the same effective range on the 20D, and there are other alternatives available. There are also DX wide angle lenses available.

Note that EF-S lenses (and Sigma/Tamron/Tokina digital lenses) cannot be used on a FF camera.

Above 100mm there is no real difference between FF and DX which is why Canon only has one telephoto EF-S lens, the 55-250. I have a 20D and all my longer lenses are FF.
--
Chris R

2manydjs said:

Hans,
Thanks for your reply, this makes a lot of sense to me!
So the lens characteristic doesn't change, but you have to use it in
a different way because of the crop with different results as a
consequence.

So using a lens on APS frame results either in: 1. cropped frames or
2. shallower DOF. Do I understand correctly?

Click to expand...

1. cropped frames or 2. GREATER DOF, not shallower.

The lens does indeed behave the same regardless of which sensor is behind it, but there is one complicating factor that is related to your final output.

In order to print or view an image from an APS sensor at the same size as the image from a FF sensor, you enlarge the APS image more. DOF is all about the size of blur that still looks sharp, and greater enlargement will enlarge the same lens-defined blur a little more and this changes the impression of in-focus for items on the margins of in/out of focus.

The effect of this is to reduce DOF a little for the smaller sensor image, though not as much as the increase in DOF from either being further away or using a shorter focal length to get the same framing as on the FF image.

So... using an APS sensor to get the same framing as an FF camera results in more DOF, but not quite as much more as you might expect!

--
Cheers,

Dave
http://purpledog.smugmug.com

'Quid quid latine dictum sit, altum videtur'

Dave Farmer said:

2manydjs said:

Hans,
Thanks for your reply, this makes a lot of sense to me!
So the lens characteristic doesn't change, but you have to use it in
a different way because of the crop with different results as a
consequence.

So using a lens on APS frame results either in: 1. cropped frames or
2. shallower DOF. Do I understand correctly?

Click to expand...

1. cropped frames or 2. GREATER DOF, not shallower.

The lens does indeed behave the same regardless of which sensor is
behind it, but there is one complicating factor that is related to
your final output.

In order to print or view an image from an APS sensor at the same
size as the image from a FF sensor, you enlarge the APS image more.
DOF is all about the size of blur that still looks sharp, and greater
enlargement will enlarge the same lens-defined blur a little more and
this changes the impression of in-focus for items on the margins of
in/out of focus.

The effect of this is to reduce DOF a little for the smaller sensor
image, though not as much as the increase in DOF from either being
further away or using a shorter focal length to get the same framing
as on the FF image.

So... using an APS sensor to get the same framing as an FF camera
results in more DOF, but not quite as much more as you might expect!

Click to expand...

The problem with this explanation, which contains some correct information, is the changing of multiple variables at the same time.

When it is stated "to get the same framing" it is implied that the shooting position is changed. So what you are describing is:
1. changing the sensor size, and
2. changing the camera shooting position.

By changing two variables simultaneously, it becomes difficult to know what to attribute the image depth of field properties.

When these two variables are isolated and changed one at a time, it can be seen that the change in sensor size (from full-frame to APS-C) results in shallower depth of field. Try it using the DOF calculator here http://www.dofmaster.com/dofjs.html

The above calculator also allows the effect of changing subject distance to be evaluated. Though of course it cannot describe the perspective properties of the image which will also change with distance.

Regards,
Peter

sherwoodpete said:

The problem with this explanation, which contains some correct
information, is the changing of multiple variables at the same time.

When it is stated "to get the same framing" it is implied that the
shooting position is changed. So what you are describing is:
1. changing the sensor size, and
2. changing the camera shooting position.

Click to expand...

Unfortunately, in practical terms that's what you do - you change sensor size and to achieve the same view you change the focal length or change shooting position. The former changes the lens properties, the latter changes focal distance and perspective.

This is the problem I've always had with crop-factor calculations. In practice, the difference between full frame and smaller sensor cameras is not one of cropping. Of course the effect of a given lens, aperture and position with different sensors is to crop to the sensor in question, but when you're looking up at Sacre Coeur from a side-street you choose your desired perspective and framing and that gives you your shooting position and focal length, not the other way around.

So, in that side street with my old film camera I chose a 50mm lens, say, now I revisit with my APS-C camera and choose a 35mm lens to achieve that framing - I don't use a 50mm and lose the edges of my image. For that reason I think you have to take multiple changes into account to achieve a practical answer as by changing just one variable you don't get the picture you want. It's a pragmatic standpoint - I don't dispute all the information relating to changing just one thing (sensor size) but in the real world, a smaller sensor camera will be using a shorter focal length and enlarging more, so all those effects are relavent.

sherwoodpete said:

By changing two variables simultaneously, it becomes difficult to
know what to attribute the image depth of field properties.
When these two variables are isolated and changed one at a time, it
can be seen that the change in sensor size (from full-frame to APS-C)
results in shallower depth of field.

Click to expand...

Absolutely true, but that's not what a photographer would be doing. To see the difference between a FF and APC camera you have to change lots of things at once.

sherwoodpete said:

Try it using the DOF calculator
here http://www.dofmaster.com/dofjs.html
The above calculator also allows the effect of changing subject
distance to be evaluated. Though of course it cannot describe the
perspective properties of the image which will also change with
distance.

Click to expand...

Cool tool.

Note that it takes account of different circle of confusion sizes for different formats - due to the differing enlargement necessary.

Actually I do realise that I've drifted off a little from the OP's question as he (or she?) was asking about the effect of changing sensor on the characteristics of a lens, but I (quite arrogantly I suppose!) feel that the intent of the question was (or could be considered to be) closer to the interpretation I have taken. If I'm wrong in this case, I apologise, but I do think that the practical difference is sometimes missed in the technical discussions.

--
Cheers,

Dave
http://purpledog.smugmug.com

'Quid quid latine dictum sit, altum videtur'

Actually I agree that your interpretation is a valid way of approaching the topic.

Perhaps it varies depending on the lens.

One lens that is commonly used on both full-frame and APS-C DSLRs is the wide-aperture 50mm prime. In this case it is probably reasonable to assume that the subject distance might be changed accordingly.

On the other hand, if a lens such as a 70 to 300mm zoom is used on each format, the camera position might not be adjusted, and the 300mm setting may well be used to gain extra magnification on APS-C.

Regards,
Peter

If you go to dof master and use the calculator, enter say

FL 50mm
Distance 3 meters
f/stop f/2

Then switch between 24x36mm and 18x24mm sensors and the DOF will be shallower on the 18x24mm.

(I understand this flies in the face of our small sensor= large DOF perceptions because that has to do with FL used on the small sensors to create a similiar field of view. If you used a common small sensor FL like 6mm lens on 24x36 you'd probably never have to bother to focus the DOF would be so large.)

Back to 24x36mm and 18x24mm DOF differences using identical situation...

Isn't the same image being cast on the focal plane just with part of the image not on the media for the smaller sensor???

Seems there must be something more to this than I understand.
--

BA baracus said:

If you go to dof master and use the calculator, enter say

FL 50mm
Distance 3 meters
f/stop f/2

Then switch between 24x36mm and 18x24mm sensors and the DOF will be
shallower on the 18x24mm.

(I understand this flies in the face of our small sensor= large DOF
perceptions because that has to do with FL used on the small sensors
to create a similiar field of view. If you used a common small sensor
FL like 6mm lens on 24x36 you'd probably never have to bother to
focus the DOF would be so large.)

Back to 24x36mm and 18x24mm DOF differences using identical situation...
Isn't the same image being cast on the focal plane just with part of
the image not on the media for the smaller sensor???

Seems there must be something more to this than I understand.

Click to expand...

The image is cast on the sensor exactly the same, but the allowable circle of confusion (CoC - the size of blur that still looks sharp) for any given final image size is smaller for the smaller sensor because you have to enlarge it more.

For an image 360mm x 240mm you enlarge a FF sensor image 10x, but the APS image is enlarges 15x to get the same final image size (a print, say).

That means that a CoC that just looks sharp on the FF enlargement now looks slightly blurred on the APS enlargement.

--
Cheers,

Dave
http://purpledog.smugmug.com

'Quid quid latine dictum sit, altum videtur'

Thanks everybody for your answers and input!

I didn't have much use for my 50mm lens on my 35mm film SLR (except for low light situations) and much preferred a 35mm lens as a "normal" lens, but use the 50mm on my Canon 50D quite a lot for portraits and short telephoto images.