[kiwi_jono]

Thought I would ask here as I honestly have not idea and a quick search has not uncovering any clear answers.
I was browsing an images EXIF and noticed that there are two sets of min / max aperture values. Example (in this case for a K-1 + FA 77 full frame image):

Min Aperture : 22
Nominal Max Aperture : 1.7
Nominal Min Aperture : 23
Max Aperture : 1.8

What is the difference between "Nominal" versus non nominal values?

[Sandy Hancock]

Moved to lens discussion forum.

[Adam]

I think the nominal values are just a rounded approximation (typically more easily recognizable) than the actual, precise values used for exposure calculation.

For example, the nominal 1/500s shutter speed on your camera is in fact technically 1/512s. Similarly, F22 is actually 22.63 and F1.8 is 1.782.

Still, it seems strange that a F1.8 lens would show up as F1.7 in the nominal field...

[DamienW]

Coincidentally, I was just looking for answers to this myself. I found a mathematical explanation, and then decided that I didn't need to know that badly.

[stevebrot]

What is the difference between "Nominal" versus non nominal values?

It is hard to say beyond pointing out that all four are in the Makernotes section of the EXIF and that the names are those pinned to these data by the ExifTool developers and interpretation is sometimes "best guess". I thought at first that MinAperture and MaxAperture are the values derived from the "A" contact coding on the lens base and that the "nominal" twins are those reported by the lens through the data contact.

Based on the EXIF from a recent shoot, the values for my FA 35/2.0 are :

Code:

Min Aperture                    : 22
Lens F Stops                    : 8.5
Nominal Max Aperture            : 2.0
Nominal Min Aperture            : 23
Max Aperture                    : 2.0

Moving to my Pentax-A 70-210/4 at 70mm, I get:

Code:

Min Aperture                    : 22
Lens F Stops                    : 8.5
Nominal Max Aperture            : 1.0
Nominal Min Aperture            : 6
(MaxAperture is missing)

and finally my Pentax-A 50/1.7:

Code:

Min Aperture                    : 22
Lens F Stops                    : 8.5
Nominal Max Aperture            : 1.0
Nominal Min Aperture            : 6
(MaxAperture is missing)

I threw in the Lens F Stops numbers because they may hold a clue. The revised theory might be that Min Aperture and Lens F Stops are derived directly from the mount contacts* and that the evil "nominal" values serve some sort of internal function with the locations possibly being used for different purposes (i.e. overloaded) depending on camera state. This is good except that there is not an 8.5 stop span for any of the lenses I sampled.** As noted by the developers, "Some of these tags require interesting binary gymnastics to decode them into useful values".

To paraphrase Inigo Montoya, "I do not think it means what we think it means."


Steve

* The mount contacts encode minimum aperture and number of f-stops.

** There are 8.5 stops from f/1.2 to f/22, the maximum number of stops possible with the base contact coding.

[stevebrot]

Moving to a lens with a min aperture of f/32 (Sigma 50/2.8 EX DG Macro)...

Code:

Min Aperture                    : 22
Lens F Stops                    : 8.5
Nominal Max Aperture            : 2.8
Nominal Min Aperture            : 32
Max Aperture                    : 2.8

I think it is safe to say that MinAperture might always be 22, Lens F Stops might always be 8.5 and that Max Aperture and the "nominals" are not-as-labeled except with an AF lens.


Steve

[kiwi_jono]

Moved to lens discussion forum.

Thanks Sandy. I realised I had posted in the wrong place but could not delete at that stage.

---------- Post added 10-26-2019 at 04:03 PM ----------

Moving to a lens with a min aperture of f/32 (Sigma 50/2.8 EX DG Macro)...

Code:

Min Aperture                    : 22
Lens F Stops                    : 8.5
Nominal Max Aperture            : 2.8
Nominal Min Aperture            : 32
Max Aperture                    : 2.8

I think it is safe to say that MinAperture might always be 22, Lens F Stops might always be 8.5 and that Max Aperture and the "nominals" are not-as-labeled except with an AF lens.


Steve

That is interesting. I had thought that maybe min / max aperture is the limits that auto exposure will use and nominal is was the actual aperture of the lens but then my FA 77 gives nominal f1.7 - f23.

---------- Post added 10-26-2019 at 04:09 PM ----------

It is hard to say beyond pointing out that all four are in the Makernotes section of the EXIF and that the names are those pinned to these data by the ExifTool developers and interpretation is sometimes "best guess". I thought at first that MinAperture and MaxAperture are the values derived from the "A" contact coding on the lens base and that the "nominal" twins are those reported by the lens through the data contact.

Based on the EXIF from a recent shoot, the values for my FA 35/2.0 are :

Code:

Min Aperture                    : 22
Lens F Stops                    : 8.5
Nominal Max Aperture            : 2.0
Nominal Min Aperture            : 23
Max Aperture                    : 2.0

Moving to my Pentax-A 70-210/4 at 70mm, I get:

Code:

Min Aperture                    : 22
Lens F Stops                    : 8.5
Nominal Max Aperture            : 1.0
Nominal Min Aperture            : 6
(MaxAperture is missing)

and finally my Pentax-A 50/1.7:

Code:

Min Aperture                    : 22
Lens F Stops                    : 8.5
Nominal Max Aperture            : 1.0
Nominal Min Aperture            : 6
(MaxAperture is missing)

I threw in the Lens F Stops numbers because they may hold a clue. The revised theory might be that Min Aperture and Lens F Stops are derived directly from the mount contacts* and that the evil "nominal" values serve some sort of internal function with the locations possibly being used for different purposes (i.e. overloaded) depending on camera state. This is good except that there is not an 8.5 stop span for any of the lenses I sampled.** As noted by the developers, "Some of these tags require interesting binary gymnastics to decode them into useful values".

To paraphrase Inigo Montoya, "I do not think it means what we think it means."


Steve

* The mount contacts encode minimum aperture and number of f-stops.

** There are 8.5 stops from f/1.2 to f/22, the maximum number of stops possible with the base contact coding.

On third thoughts this sheds further light. The A 50/17 is particularly interesting.

My current theory is that these aperture readings are based off communications from the lens itself - because the A 50 f1.7 only has the mount contacts (no data communications from the lens in this case) and possibly the shown information is just default values.

---

Last edited by kiwi_jono; 10-25-2019 at 08:11 PM.

[stevebrot]

My current theory is that these aperture readings based off communications from the lens itself - because the A 50 f1.7 is based only off the mount contacts (no data chip in the lens in this case) and possibly this information is default values.

Here are a few that you did not ask about (from my FA 35/2.0)...

Code:

AE Aperture                     : 2.8
AE Max Aperture                 : 2.0
AE Max Aperture 2               : 2.0
AE Min Aperture                 : 23

(The shot was at f/2.8 in Av mode)


Steve

[kiwi_jono]

Here are a few that you did not ask about (from my FA 35/2.0)...

Code:

AE Aperture                     : 2.8
AE Max Aperture                 : 2.0
AE Max Aperture 2               : 2.0
AE Min Aperture                 : 23

(The shot was at f/2.8 in Av mode)


Steve

Thanks Steve. Those parameters are not there for the FA 77 & FA 43.

BTW FA 43:

Auto Aperture : On
Min Aperture : 22
Nominal Max Aperture : 2.0
Nominal Min Aperture : 23
Max Aperture : 1.9
Aperture : 8.0

[UncleVanya]

Could it be that the nominal minimum value is the value calculated with the passive lens contacts? I know there are only a set number of options and f1.7 is as close to 1.8 as you can get. F 1.9 also can't be represented. The nominal max value however is odd.

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