Shallow DOF in 50 mm affected by 28 mm lens?

[Daneinbalto]

From what is said, shallow depth of field is less easily obtained on a 28 mm lens than a 50 mm lens. But what happens to DOF if you apply a 50 mm crop to a shot captured with a 28 mm lens? If you compare two shots taken of the same subject, at the same distance, with the same f/stop, one with a native 50 mm lens and the other with a 28 mm lens but cropped to correspond to the 50 mm shot, will the DOF be different? Someone seemed to think the depth of field would be greater in the latter situation because the shot originated from a 28 mm lens.

 

[benmacphoto]

I believe the DOF will be different if you take a shot with a 28mm lens and crop it to correspond to a 50mm lens shot with the same f stop and same distance.

 

[ColSebastianMoran]

Think about it this way: DOF isn't mysterious, it's a function of the size of the aperture as seen by the subject. Taking in rays coming from different angles, therefore bluring the background, that produces the OOF in the background.

So, your 28mm lens at f/2 has a 14mm aperture. Make the shot and crop, it still has a 14mm aperture. At half size, that will be like a 50mm lens at f/4.

Your 50mm lens at f/2 has a 25mm aperture. Maybe not exactly, but close enough.

So, the cropped image from the 28mm f/2 will have more DOF than the 50mm at f/2.

Does that help?

More: Depth of Field is subjective, depends on lots of things, it's not just a technical consideration.

 

[aizan]

http://toothwalker.org/optics/dof.html

read this!

 

[TXForester]

So, the cropped image from the 28mm f/2 will have more DOF than the 50mm at f/2.

This is how I understand it.

But you can take the cropped image and the uncropped image and make prints of the same size, and view them from the same distance, then the perceived depth of field from the cropped image shrinks. Though it may not look equivalent to the uncropped print.

 

[Rob-F]

This is how I understand it.

But you can take the cropped image and the uncropped image and make prints of the same size, and view them from the same distance, then the perceived depth of field from the cropped image shrinks. Though it may not look equivalent to the uncropped print.

I agree with this. DOF is a zone of acceptable unsharpness. Cropping and enlarging magnifies the image that remains after cropping, and the unsharpness gets magnified along with it. So what was acceptable unsharpness in the uncropped image can become unacceptable unsharpness after cropping and magnifying.

 

[teddy]

I think also the diameter of the lens opening - not the aperture but the actual size of the element on the front of the lens will affect the size of the circle of confusion co-relating to the depth of field and angle effect of the image. Correct me if I'm wrong anyone?

 

[peterm1]

The depth of field is determined by the lens optical characteristics. Cropping a shot taken with a 28mm lens to look like a 50mm does not change the DOF as it cannot - the DOF is something that is determined by factors such as the focal length of the lens, the f stop used, whether it was designed for a full frame or other camera, the distance to the point of focus etc. All of these are fixed at the moment the shutter button is pressed. Nothing you do with the shot afterwards changes it or possibly can change it. The image is already in the camera.

Also based on the above if you take a 28mm lens from a full frame camera and put it on a small format camera using an adapter it will have the same DOF (with minor variations) on either camera. (All the smaller camera is doing in this case is cropping the image because the sensor is smaller ( it does not change thee characteristics of the lens.)

But if you take a 28mm lens designed for a small format camera and keep all of those other factors the same it would be expected to have a different DOF from a 28mm designed for a full frame camera as optically its different. Other things being equal, DOF is determined largely by the size of the aperture being used and the fact is that lenses designed for such a camera are smaller than those designed for their larger format brothers. This results in a larger DOF at each distance, each f stop etc.

 

[Scrambler]

I think also the diameter of the lens opening - not the aperture but the actual size of the element on the front of the lens will affect the size of the circle of confusion co-relating to the depth of field and angle effect of the image. Correct me if I'm wrong anyone?

Sorry, you are wrong. It's the effective aperture versus the lens strength. The most limiting lens is usually inside.

 

[Scrambler]

Think about it this way: DOF isn't mysterious, it's a function of the size of the aperture as seen by the subject. Taking in rays coming from different angles, therefore bluring the background, that produces the OOF in the background.

So, your 28mm lens at f/2 has a 14mm aperture. Make the shot and crop, it still has a 14mm aperture. At half size, that will be like a 50mm lens at f/4.

Your 50mm lens at f/2 has a 25mm aperture. Maybe not exactly, but close enough.

So, the cropped image from the 28mm f/2 will have more DOF than the 50mm at f/2.

Does that help?

More: Depth of Field is subjective, depends on lots of things, it's not just a technical consideration.

This is pretty close - though as has been said the enlargement of the image may alter the DOF issues - usually exacerbate them not reduce them - it's more common for slightly out of focus images to LOOK in focus with smaller magnifications.

 

[peterm1]

This is pretty close - though as has been said the enlargement of the image may alter the DOF issues - usually exacerbate them not reduce them - it's more common for slightly out of focus images to LOOK in focus with smaller magnifications.

I agree with this. Its what I meant in my post about "minor variations". ie a bigger enlargement (e.g. to get the same sized print from a smaller format camera/ sensor) will result in a marginally smaller DOF as it exacerbates any blur in the image

 

[tunalegs]

It's quite easy to visualize this. A medium format camera, and a 110 camera, both having lenses giving the same angle of view for the respective formats, would have vastly different results in depth of field.

 

[icebear]

DOF with a 28mm lens will be deeper than with a 50mm at the same aperture shot at the same distance.
If you get pretty close, you can achieve shallow DOF also with a 28.
Q :

 

[mfogiel]

Just use this and compare:
http://www.dofmaster.com/dofjs.html

 

[grouchos_tash]

This works the same way on smaller sensors. Thats's why you get more DOF on an Olympus pen with a 25mm f2 lens than a Sony A7 with a 50mm f2 🙂

 

[jsrockit]

Are we talking about the Q? If you crop to 50mm (in camera), it is still a 28mm lens that is cropped. Same depth of field.

 

[teddy]

Sorry, you are wrong. It's the effective aperture versus the lens strength. The most limiting lens is usually inside.

Ok, I stand corrected, but wouldn't the front pupil affect the character (or the look) of the depth of field? This is probably besides the point.

 

[Daneinbalto]

Empirical data needed

Empirical data needed

Would anyone care to do the experiment? Take a shot with a 50 mm lens at f/1.4 of a scene well suited to illustrate DOF. Then take the same shot (from the same vantage point and with the same camera) with a 28 mm lens at f/1.4. Crop the 28 mm shot to correspond to the 50 mm shot. Enlarge the crop to have the same size as the native 50 mm shot and let us compare. That would illustrate how much, if at all, the DOF of the 50 mm crop from the Q differs from a shot using a native 50 mm lens. For a proof of principle, the lenses don't have to be exactly 28 mm and 50 mm. If the effect is there it will be exaggerated if comparing two lenses that are further from each other in focal length.

 

[jsrockit]

Would anyone care to do the experiment? Take a shot with a 50 mm lens at f/1.4 of a scene well suited to illustrate DOF. Then take the same shot (from the same vantage point and with the same camera) with a 28 mm lens at f/1.4. Crop the 28 mm shot to correspond to the 50 mm shot. Enlarge the crop to have the same size as the native 50 mm shot and let us compare. That would illustrate how much, if at all, the DOF of the 50 mm crop from the Q differs from a shot using a native 50 mm lens.

Of course it is different. A 28mm lens has more depth of field at a given aperture than a 50mm lens. A 50mm crop function on a Leica Q is still made from a 28mm lens. The crop has a 28mm depth of field it's just cropped to a 50mm field of view.

 

[Daneinbalto]

Of course it is different. A 28mm lens has more depth of field at a given aperture than a 50mm lens. A 50mm crop function on a Leica Q is still made from a 28mm lens. The crop has a 28mm depth of field it's just cropped to a 50mm field of view.

That's nice but it's the same theoretical argument that has been stated before, not an empirical one.