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Different pixel count, different focus setting?

Pemihan

Well-known member
If you take two backs lets say an 80mp and 100mp. Would the focus at infinity be different?

I have my tech camera lenses (Cambo) calibrated to my 80mp back so focus is spot on wide open at infinity. When testing a 100mp back I found that I needed to dial the focus a little closer in order to achieve the same.

Both backs are Phase One, so I assume they are calibrated to the same standard (or at least they should) so I'm a bit puzzled about this. Can the difference in pixel density account for this?

Peter
 
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ejpeiker

Member
Yes and no ;) There's nothing about the pixel count difference that inherently changes the focus point but the two backs could have slightly different mount to sensor distances due to manufacturing tolerances that could impact focus.
 
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Aviv1887

Member
What I have noticed several times when I've changed DB's over the years, is that I did have to adjust for infinity. I work with an Alpa system where you do have to adjust or double check the shims for infinity. I've noticed in particular when I changed from an IQ260 to an IQ3100 that the shims needed to be adjusted quite a lot. Previously it was very minimal. That said, I'm not surprised that you have noticed the difference and imho it's with the DB and not the camera system. These systems are so crazy precise in alignment and focusing, in particular when you go up in MP's. (although 80-100 is not that much).
 

Pemihan

Well-known member
.....but the two backs could have slightly different mount to sensor distances due to manufacturing tolerances that could impact focus.
I get that, but since the backs are focus calibrated at Phase One it should be eliminated given all the backs are calibrated to the same standard.
 

Pemihan

Well-known member
What I have noticed several times when I've changed DB's over the years, is that I did have to adjust for infinity. I work with an Alpa system where you do have to adjust or double check the shims for infinity. I've noticed in particular when I changed from an IQ260 to an IQ3100 that the shims needed to be adjusted quite a lot. Previously it was very minimal. That said, I'm not surprised that you have noticed the difference and imho it's with the DB and not the camera system. These systems are so crazy precise in alignment and focusing, in particular when you go up in MP's. (although 80-100 is not that much).
Wonder if after the change from the DF to the XF the backs are calibrated to a slightly different standard? Anyhow if you have two backs that require slightly different adjustment for infinity you have to adjust infinity for the back where the infinity point is sharpest furthest away. Otherwise you will run out of infinity with the other back.
 

dchew

Well-known member
Wonder if after the change from the DF to the XF the backs are calibrated to a slightly different standard? Anyhow if you have two backs that require slightly different adjustment for infinity you have to adjust infinity for the back where the infinity point is sharpest furthest away. Otherwise you will run out of infinity with the other back.
A 100mp back is so unforgiving. I used to think shimming the Alpa was good enough, but not anymore. I've gone through each lens to find the one that is front focused the most, then shimmed the back to that one. For all the others I set an "offset" into the HPF rings to account for the differences. In my case, the 90hr-sw is the one I shimmed to. My 40hr, 60xl and sk150 all focus a bit beyond infinity.

Dave
 

Shashin

Well-known member
I get that, but since the backs are focus calibrated at Phase One it should be eliminated given all the backs are calibrated to the same standard.
But standards have tolerances. If you are looking at 100%, then a higher resolution back would need greater tolerances for depth of focus (the area around the image plane). Slight variations in manufacturing would add to that. And then there is your Cambo calibration which might be fine for a larger pixel pitch, but would add to the difference in a smaller pixel pitch (granted the pixel pitch difference is not that much, so I image the mechanical tolerances are greater). It seems that the combined tolerances are making a difference in your system. Perhaps add shims to one of the backs.
 

vjbelle

Well-known member
When I got my 3100 and still using an STC I said goodby to all shims other than to shim once to make sure that all lenses would focus beyond infinity. I would never trust the infinity stops on the lenses used for MF because they are prone to be different making shimming very error prone. I always focus with a loup so there is/was no need for HPF rings. I find that the ability to focus beyond infinity makes it easier to accurately focus for infinity. Now, working exclusively with an Actus, I find being able to focus beyond infinity essential for accurate focus.

This works well for me.....YMMV.

Victor
 

Pemihan

Well-known member
With a Cambo you cant shim/adjust the individual back so if you were to have two backs with slightly different focus tolerances it seems the best bet is to adjust the lenses to focus slightly beyond infinity or at least adjust for the back where infinity is furthest away. Unless of course you can have the backs calibrated to have exactly the same focus setting.
 

ErikKaffehr

Well-known member
Hi,

As far as I know, Phase One's tolerances are +/- 12 microns on the sensor. So, maximum variation between backs should be 24 microns. The pixel pitch on the 100 MP sensor is around 4.6 microns. It is reasonable to assume that you may be able to observe a loss of sharpness if the circle of confusion (CoC) is larger than the pixel pitch.

If you look at the image side, the CoC is defocus / aperture. So, maximum tolerance is 24 microns and pixel pitch is 4.6 microns. That means that you should not be able to observe change of sharpness if stopped down beyond 5.2. So, if you see difference in focus at f/8, it means that one of your backs is outside tolerance. If you can notice a difference at f/4, both backs can be within tolerance.

To put things in perspective, the Hasselblad V system had a factory tolerance of 30 microns, so Phase One tolerance is quite a bit tighter.

If you have your backs under warranty, you may consider sending them in for service if you can see a difference in focus at f/8 or smaller.

Best regards
Erik






If you take two backs lets say an 80mp and 100mp. Would the focus at infinity be different?

I have my tech camera lenses (Cambo) calibrated to my 80mp back so focus is spot on wide open at infinity. When testing a 100mp back I found that I needed to dial the focus a little closer in order to achieve the same.

Both backs are Phase One, so I assume they are calibrated to the same standard (or at least they should) so I'm a bit puzzled about this. Can the difference in pixel density account for this?

Peter
 

Pemihan

Well-known member
Thanks Erik...

Hi,

As far as I know, Phase One's tolerances are +/- 12 microns on the sensor. So, maximum variation between backs should be 24 microns. The pixel pitch on the 100 MP sensor is around 4.6 microns. It is reasonable to assume that you may be able to observe a loss of sharpness if the circle of confusion (CoC) is larger than the pixel pitch.

If you look at the image side, the CoC is defocus / aperture. So, maximum tolerance is 24 microns and pixel pitch is 4.6 microns. That means that you should not be able to observe change of sharpness if stopped down beyond 5.2. So, if you see difference in focus at f/8, it means that one of your backs is outside tolerance. If you can notice a difference at f/4, both backs can be within tolerance.

To put things in perspective, the Hasselblad V system had a factory tolerance of 30 microns, so Phase One tolerance is quite a bit tighter.

If you have your backs under warranty, you may consider sending them in for service if you can see a difference in focus at f/8 or smaller.

Best regards
Erik
 
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