[filmamigo]

Hi folks - I ran across an interesting comment in the lens reviews for the Pentax-A 20mm. Rather than reply to the original poster, I thought I would ask everybody to try and explain the comment.

@torashi ... Make sure your copy [of the lens] has a calibrated aperture (only relevant if you're shooting it in the A position).

I always assumed that Pentax-A lenses were similar to Nikon Ai-S lenses, with the aperture mechanism linearized and optimized for automatic (aperture priority or program) operation.

Can anyone elaborate on what the specific concern is? Is calibrating apertures something done with a lens service? Is this only applicable to Pentax-A lenses, or to all A, F, FA and above?

Please tell me anything you can about calibrated apertures!

Thanks

[pschlute]

Hi folks - I ran across an interesting comment in the lens reviews for the Pentax-A 20mm. Rather than reply to the original poster, I thought I would ask everybody to try and explain the comment.

@torashi ... Make sure your copy [of the lens] has a calibrated aperture (only relevant if you're shooting it in the A position).

I always assumed that Pentax-A lenses were similar to Nikon Ai-S lenses, with the aperture mechanism linearized and optimized for automatic (aperture priority or program) operation.

Can anyone elaborate on what the specific concern is? Is calibrating apertures something done with a lens service? Is this only applicable to Pentax-A lenses, or to all A, F, FA and above?

Please tell me anything you can about calibrated apertures!

Thanks

The only thing I can think of is he means to make sure the aperture is working normally....ie it is snappy at closing and opening again. You can test this by flicking the aperture lever at the back of the lens. Some Pentax lenses from this era did tend to suffer occasionally from lazy or sticky blades, which would need a service/clean to get them working properly again. I have no idea if it is a potential issue with this specific lens

[UncleVanya]

Non-a series lenses didn’t have calibrated movement that was standardized and so the cameras could not control them accurately without setting the aperture on the lens. A series lenses are designed to have a proportional movement of the aperture to the lever which is why you can select the aperture from the body. There’s no specific calibration needed if the lens and body are working. If the either has been damaged or otherwise modified the aperture might be inaccurate. Also metering with “a series” lenses can be a challenge in some situations with certain methods - but that is a complex question that I’m not the expert on.

[Lowell Goudge]

Hi folks - I ran across an interesting comment in the lens reviews for the Pentax-A 20mm. Rather than reply to the original poster, I thought I would ask everybody to try and explain the comment.

@torashi ... Make sure your copy [of the lens] has a calibrated aperture (only relevant if you're shooting it in the A position).

I always assumed that Pentax-A lenses were similar to Nikon Ai-S lenses, with the aperture mechanism linearized and optimized for automatic (aperture priority or program) operation.

Can anyone elaborate on what the specific concern is? Is calibrating apertures something done with a lens service? Is this only applicable to Pentax-A lenses, or to all A, F, FA and above?

Please tell me anything you can about calibrated apertures!

Thanks

I think it is a load of BS.

As other posters have explained, the lens should have a linear movement of stops vs position so that the body controls the aperture correctly, however having said that, you can and should check exposure linearity of the lens/camera combo, by taking a sequence of shots at a uniform surface (I use block walls, sidewalks or paved roads) with each and every aperture and then measure the average greyscale value of the shots. It should be close to 128

Although most lenses will produce a very flat exposure vs aperture, sometimes you can have errors in the forst or last stop positions. I have also experienced one lens,[Tamron XR di 28-75/2.8) that had an exposure error drifting from 1/2 stop over exposed wide open to 1/2 stop under exposed when stopped down.

I have also with some bodies, notably K10 and K1MKII , where addition of a teleconverter requires some exposure adjustment because the reported aperture is not the same as the effective aperture. The K10 was notorious for this, and I found the K1 was similar. The *istD, K7 and K5 did not have this issue.

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Last edited by Lowell Goudge; 12-19-2022 at 01:35 PM.

[Digitalis]

I think it is a load of BS.

Agreed, this is a bunch of hand waving nonsense. In respect to what frame of reference are they calibrating the aperture to exactly*? If the camera body is behaving in a non linear fashion a calibrated lens isn't going to improve anything. In respect to linearity, a well serviced lens coupled with a well serviced and maintained camera will produce excellent results in respect to their respective performance envelopes, nothing more.


* and more importantly, how are they calibrating the aperture?

[JensE]

From the 20 or so Pentax lenses with mechanical coupling that I use, none has any noticeable calibration issues and I also haven't read about specific problems with the Pentax-A 20mm. Two third-party lenses though from Irix and Laowa are not well behaved in the first step (fully open -> stopped down one step), but not as much as would be enough of a reason for me to have them serviced.

[Andrea K]

Isn't BS: Problem with smc-A 20mm f/2,8 and K-1 - PentaxForums.com
More specifically: https://www.pentaxforums.com/forums/10-pentax-slr-lens-discussion/374243-pro...ml#post4434995

[Just1MoreDave]

Lenses with an aperture ring and an A setting have a mechanism for moving the blades when you use the aperture ring, which isn't used at all when the lens is in the A setting. Effectively the lens works like an K/M series when you use the aperture ring, and an A series when you don't. I suppose a clever mechanical person or real lens technician could get these systems to agree perfectly. It would only be useful if you always needed both systems to agree, or had OCD. I have not seen an adjustment system on the cheapy A series lenses I'm familiar with, though the high-end stuff might be better.

[UncleVanya]

Isn't BS: Problem with smc-A 20mm f/2,8 and K-1 - PentaxForums.com
More specifically: Problem with smc-A 20mm f/2,8 and K-1 - PentaxForums.com

What’s missing is: Did you get this fixed on your lens? And how was it accomplished? Or is this just the way it works? Are other lenses of the exact same type exhibiting different issues suggesting calibration can fix or are all of the same lens models just like this, implying an error in design.

[Lowell Goudge]

Isn't BS: Problem with smc-A 20mm f/2,8 and K-1 - PentaxForums.com
More specifically: Problem with smc-A 20mm f/2,8 and K-1 - PentaxForums.com

I will admit I only scanned the two linked posts, BUT

For the first, the user wanted to set the lens to hyper focal distance, and then complains the aperture is not closing correctly. Forgive me but how does the camera know what the user is even thinking??. Hyper focal focusing is a totally manual function


For the second, I suspect the user was discovering the difference between green button metering and automatic metering. The green button can be highly inconsistent in exposure especially for fast lenses.

As I explained in my post it is always good practice to check each and every lens you get, for exposure control and linearity.

[Andrea K]

The user is me.
Hyperfocal is a combination of focus distance and aperture and the correct values are engraved on the barrel, so....
The color differences were due to the different white balance if the lens isn't in A position, obliviously exposure values are the same in every shots (Problem with smc-A 20mm f/2,8 and K-1 - PentaxForums.com)

---------- Post added 21-12-22 at 18:10 ----------

What’s missing is: Did you get this fixed on your lens? And how was it accomplished? Or is this just the way it works? Are other lenses of the exact same type exhibiting different issues suggesting calibration can fix or are all of the same lens models just like this, implying an error in design.

No fix for the lens, I can't find anyone that is able to work in this old lens. When I need the hyperfocal I use the ring to set the aperture to f8.

[UncleVanya]

The user is me.
Hyperfocal is a combination of focus distance and aperture and the correct values are engraved on the barrel, so....
The color differences were due to the different white balance if the lens isn't in A position, obliviously exposure values are the same in every shots (Problem with smc-A 20mm f/2,8 and K-1 - PentaxForums.com)

---------- Post added 21-12-22 at 18:10 ----------



No fix for the lens, I can't find anyone that is able to work in this old lens. When I need the hyperfocal I use the ring to set the aperture to f8.

Which suggests calibration is not an offered service. Damage to the lens or camera body or just bad design seems possible.

[Andrea K]

Which suggests calibration is not an offered service. Damage to the lens or camera body or just bad design seems possible.

Yep, they spoke about no more spare parts for this lens

[UncleVanya]

I will admit I only scanned the two linked posts, BUT

For the first, the user wanted to set the lens to hyper focal distance, and then complains the aperture is not closing correctly. Forgive me but how does the camera know what the user is even thinking??. Hyper focal focusing is a totally manual function


For the second, I suspect the user was discovering the difference between green button metering and automatic metering. The green button can be highly inconsistent in exposure especially for fast lenses.

As I explained in my post it is always good practice to check each and every lens you get, for exposure control and linearity.

The main point was that using the aperture ring and using the lens in “A” with the body controlling the aperture doesn’t give the same results. The depth of field is different and the size of the opening is different. Clearly there’s an error. What’s not clear is if this is an error due to this copy being made wrong, worn out, damaged, or even designed wrong; or if there’s an issue with the body or the design of the “A” mechanism related to the model of body. The only thing clear is there is a mismatch between the f stops with this specific lens depending on how it’s selected (“A” vs ring).

[Lowell Goudge]

Have you checked and / or cleaned all the aperture contacts? The camera is designed to interpolate uniformly between the min and max aperture based upon the coding of the aperture contacts. If there is intermittent or poor contact on one of the pins it would get this wrong.

The way to test this, which you should do for all your lenses any way, is to run a series of shots with green button metering for manual aperture, and using aperture control for the A position, and plot the measured average grey scale value for both shots. Your target should be a uniform and uniformly lit grey surface. I like paved roads, sidewalks and /or concrete block walls.


You should also check that there is no ev compensation set for auto exposure. Also use the same metering for both, which should be either spot or Center weighted.