Tips on Zone Focusing

Dof

Dof

The lens characteristic does not change with the body, that is true. But dof is not just a measurement, like focal length or filter size. The idea is to have an acceptable sharpness between two distances in a photo. What is acceptable is based on a given value of the circle of confusion. However, when one increases the crop factor, thereby increasing its effective focal length, the original calculations no longer hold. Hence the perception that the image is not so sharp. The lens didn't change, but you've blown up the centre of the photo by using a smaller than 35mm sensor. How can you expect the dof scales on a lens to hold?


batterytypehah! said:
Not so. The DOF of a given focal length at a given aperture doesn't change. That's the whole crux with adapted legacy lenses on crop factor bodies. The FOV changes according to the crop factor, but the DOF remains the same. Which is why lenses that are fast for their FL are so sought after now.

As an example, if you put a 28mm lens on a Micro 4/3 body (crop factor of 2), it will act like a "normal" lens in terms of its FOV, equivalent to 56mm on 135 film. But it's still a 28mm lens with its really deep DOF, and blurring the background will be much harder than with a normal lens on film.
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cpc said:
Just shim the adapter. The problem is not in the change of FOV on a crop body. Your adapter is most likely too thin. I've seen only one adapter in my life which was perfect thickness out of the box, and I have dozens ...
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I agree, although I haven't had the opportunity to use that many adapters.

However, I think that these are being produced (pushed out) so quickly that I would be concerned about quality control and whether spot checks are made to ensure that they meet the specs.

If you have loose tolerance in one adapter and loose tolerance in a second adapter, you can really end up with a wide margin of error when you combine the two.
 
waileong said:
The lens characteristic does not change with the body, that is true. But dof is not just a measurement, like focal length or filter size. The idea is to have an acceptable sharpness between two distances in a photo. What is acceptable is based on a given value of the circle of confusion. However, when one increases the crop factor, thereby increasing its effective focal length, the original calculations no longer hold. Hence the perception that the image is not so sharp. The lens didn't change, but you've blown up the centre of the photo by using a smaller than 35mm sensor. How can you expect the dof scales on a lens to hold?
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I'm afraid you're confusing film and digital. It is not "blowing up" that's happening here. When you enlarge a negative, you're enlarging the grain along with it. With a digital sensor, you get 12 megapixels (I believe, for the camera the OP is using) on a chip half the size of a 135 negative. Not the same thing at all.
 
Really? Why?

What has the no of pixels got to do with it?

You're cropping the image, that changes the whole basis of the dof calculations.

So why do you think the dof marks on the lens are still effective?


batterytypehah! said:
I'm afraid you're confusing film and digital. It is not "blowing up" that's happening here. When you enlarge a negative, you're enlarging the grain along with it. With a digital sensor, you get 12 megapixels (I believe, for the camera the OP is using) on a chip half the size of a 135 negative. Not the same thing at all.
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I've been told, when using a FF lens at f/8, lets say for argument sake, on a 1.5x camera...
to use the DoF scale for f/11 while shooting at f/8, since the f/8 DoF on a FF is similar to f/11 of the similar Focal Length FoV, on the 1.5x Camera.

The F/stop light gathering remains the same... but, the perceived DoF is altered?

Haven't really tested this... I always manually focus with adapted lenses.
 
I run into such situations, and that is why I posted this thread. Your comments are quite interesting.

The problem of incorrectly focusing is really only when you don't want to focus with the camera finder or back, and you want to zone guess the focus.
 
waileong said:
Really? Why?

What has the no of pixels got to do with it?

You're cropping the image, that changes the whole basis of the dof calculations.

So why do you think the dof marks on the lens are still effective?
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For the sake of this discussion, I'll assume you're not being combative here, but I would advise you to watch the tone of what you post.

So, taking your question at face value, I think so because of personal experience. As I pointed out many posts ago, I use adapters for several legacy systems on my own M4/3, in my case an E-PL1, very similar to Raid's camera. The only native lens I use is the 15/f8 "body cap," everything else is PK, Konica and LTM. I ran into exactly the same problem Raid is having and I can assure you and everybody else in this thread, go and measure your adapter before you blame anything else. I mean, do you really expect that for 8 bucks shipped all the way from China, through a couple of middlemen, you are going to get something that's machined to 1/10 of a mm precision? Luck of the draw if you do.

As for DOF of a lens across frame sizes, that is a red herring in the context of the OP, and has been discussed over and over, and not just in the film vs digital age. It applies to all image formats, except with film you had the extra complication that the grain of a given emulsion was always the same absolute size, which is not true for pixel sites on a sensor.

Consider also that by cropping the image circle of the lens, you're cutting off the weakest performing part. Lenses are typically best in the center.

In practice, scale focusing on a crop camera is easier than with film, because you have more DOF at the same effective FL.

All that said, it's always wise to know if the DOF scale of your lens can be trusted, and give yourself one extra stop if you can. That's nothing new at all.
 
There's nothing in your post that explains why the dof scales are still effective.

Saying that the centre of the lens is the best part is a red herring. Dof calculations are not dependent on the lens. In fact, I don't think it depends on grain size either, since dof calculations do not specify any film emulsion.

I appreciate your point about quality adapters, but I'm not the one using adapters. I'm only pointing out that, if the effective focal length of a lens is increased by a crop factor, its dof logically follows that of the increased focal length.

You talk about your personal experiences, but do not explain, in scientific or mathematical terms, why with digital "you have more DOF at the same effective FL". This is counter-intuitive and cannot just be ascribed to your personal experiences.

batterytypehah! said:
For the sake of this discussion, I'll assume you're not being combative here, but I would advise you to watch the tone of what you post.

So, taking your question at face value, I think so because of personal experience. As I pointed out many posts ago, I use adapters for several legacy systems on my own M4/3, in my case an E-PL1, very similar to Raid's camera. The only native lens I use is the 15/f8 "body cap," everything else is PK, Konica and LTM. I ran into exactly the same problem Raid is having and I can assure you and everybody else in this thread, go and measure your adapter before you blame anything else. I mean, do you really expect that for 8 bucks shipped all the way from China, through a couple of middlemen, you are going to get something that's machined to 1/10 of a mm precision? Luck of the draw if you do.

As for DOF of a lens across frame sizes, that is a red herring in the context of the OP, and has been discussed over and over, and not just in the film vs digital age. It applies to all image formats, except with film you had the extra complication that the grain of a given emulsion was always the same absolute size, which is not true for pixel sites on a sensor.

Consider also that by cropping the image circle of the lens, you're cutting off the weakest performing part. Lenses are typically best in the center.

In practice, scale focusing on a crop camera is easier than with film, because you have more DOF at the same effective FL.

All that said, it's always wise to know if the DOF scale of your lens can be trusted, and give yourself one extra stop if you can. That's nothing new at all.
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If I may refer you to Wikipedia.

"Same focal length for both formats
Many small-format digital SLR camera systems allow using many of the same lenses on both full-frame and “cropped format” cameras.

1. If, for the same focal length setting, the subject distance is adjusted to provide the same field of view at the subject, at the same f-number and final-image size, the smaller format has greater DOF.

2a. If pictures are taken from the same distance using the same f-number, same focal length, and the final images are the same size, the smaller format has less DOF.

2b. If pictures taken from the same subject distance using the same focal length, are given the same enlargement, both final images will have the same DOF. The pictures from the two formats will differ because of the different angles of view.

3. If the larger format is cropped to the captured area of the smaller format, the final images will have the same angle of view, have been given the same enlargement, and have the same DOF."


1) means that you shoot closer with full frame than with cropped sensor, to give the same field of view. Obviously when the subject distance is closer, the dof is smaller with full frame.

2a) means fixing the subject distance, so the cropped sensor will show a larger subject due to the crop factor. As stated, the dof is smaller for the cropped lens.

2b) means that if we crop the images from the full frame to match the cropped sensor, the dof of both images are the same-- perfectly logical.

3) reinforces 2b above, by saying if we crop the full frame image to the same angle of view as the cropped sensor, both images will have the same dof.

The conclusion from the above is quite simple: for the same subject distance, the cropped sensor image will have a smaller dof than the full frame image. That's why the dof scale, which was engraved with 35 mm in mind, is no longer valid on a cropped sensor. This has nothing to do with pixels or film grain size, it's just mathematics.

On the other hand, if the full frame image is cropped to match the angle of view of the cropped sensor, and enlarged to the same size, then both images have the same dof. Again, this has nothing to do with film grain, it's mathematics.
 
I'm not here to explain basic optics to you. I'm here to help Raid figure out why his pictures were out of focus. Which, I believe, I have done.
 
Thanks for all the tips and comments. There should always be room to disagree on opinions.
 
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