[Vicioustuna2012]

So, this should be a simple question to answer. I have a Quantaray 28-90mm lens that's f22 at it's smallest aperture, but when shooting in Av or manual, whatever, my k-30 will allow me to set the aperture all the way to 40. Is something wrong, or is this normal? If it is normal, will me setting the aperture higher than the lens throw off the metering, or does it just revert to the highest capable setting of the lens?

[UncleVanya]

I suspect your lens has a variable minimum and maximum aperture that depends on focal length. If the lens is on 90mm you can set it to f/40 but if you zoom to 28mm I'll bet it drops back to f/22. Aperture rings on lenses like that only show the widest numbers that can be achieved and not the effect of longer focal length.

Some lenses have the same min/max aperture at all focal lengths but that is typically more expensive.

---------- Post added 11-26-17 at 03:38 PM ----------

This variability is from the math, the aperture diameter on these variable lenses stays the same at a given setting, but as focal length changes this changed what the f/stop is at that setting. Fixed lenses effectively open wider as the lens zooms to longer focal lengths keeping f stop unchanged.

[stevebrot]

I have a Quantaray 28-90mm lens that's f22 at it's smallest aperture, but when shooting in Av or manual, whatever, my k-30 will allow me to set the aperture all the way to 40

Is the a manual focus lens or auto-focus? The body reads minimum aperture in the following two ways:

• The standard "A" contacts on the mount face (available for KA and all auto-focus mounts). In this case, the minimum aperture is a choice of f/16, f/22, f/32, or f/45 depending maximum aperture (See chart at KMP, middle of page LINK)
• The data contact found on KAF and more recent AF mount variants. The values available to the body are read from the lens by means of that contact.

With variable-aperture zooms the values read using either means may change with focal length. In your case, the f/40 number sounds a bit strange. That number is not in the sequences for full, 1/2, or 1/3 stop increments. Assuming yours is one of the 28-90 f/3.5-5.6 AF Quantarays, I would expect a shift to f/32 or f/36 at the long end with possible intermediate settings at f/25, f/27, (32), and f/29 depending how the camera is set up.

I will do a bit of look-see with the variable-aperture zooms on my shelf to try and replicate.


Steve

[stevebrot]

I will do a bit of look-see with the variable-aperture zooms on my shelf to try and replicate.

I mounted up the DA 18-55/3.5-5.6 (same maximum aperture range) to my K-3 (your K-30 should behave the same) and found that:

• 18mm --> min aperture = f/22
• 55mm --> min aperture = f/40 (!)

But wait! There's more! It occurred to me that only the maximum aperture is read from the lens via the data pin and that the minimum is calculated from the known range of travel for the aperture actuator on the body. I did a quick series at f/32, f/36, and f/40 (keeping constant shutter speed for all three) and found that the f/36 and f/40 exposures were identical. In short, the aperture actuator was likely not able to run full travel to f/40, but stopped at the very logical f/36.

I don't know whether shooting at f/40 will risk damage to lens or body, though there is probably little sense in intentionally shooting at that setting or even f/36.

Correction: Actually, I do know. The aperture coupling on the lens passively follows the body lever. The minimum aperture on the lens is simply the point at which the coupling lever stops. There is zero risk to lens or body, just the possibility of a small (less than 1/3 stop) overexposure.


Steve

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Last edited by stevebrot; 11-26-2017 at 02:40 PM.

[mikesbike]

Are you setting aperture via an aperture ring on the lens? With such a lens, the control is mechanical, not electronic, so the aperture must change in a variable-aperture zoom lens as it is being zoomed. The camera's readout is correct, not the number you have set on the aperture ring. If the aperture is set electronically on the camera body, not by using an aperture ring, the camera body will control the aperture, which will not change, unless it is set to a wide-open maximum aperture at the shorter part of the zoom range, which the camera body cannot electronically maintain as you zoom to a loner FL, since that would make the lens a faster one than it is.

OTOH, you could actually set a higher aperture than rated like f/40 electronically with a variable-aperture lens, if it has the electronic contacts, but that aperture will only be available at the longer zoom ranges, where the aperture would shift to if it were set mechanically at f/22. If the lens is zoomed all the way back to its shorter end where maximum is say f/3.5, it will have the stated range of f/3.5-22, not f/3.5-f/40 at that focal length.

[Vicioustuna2012]

I mounted up the DA 18-55/3.5-5.6 (same maximum aperture range) to my K-3 (your K-30 should behave the same) and found that:

• 18mm --> min aperture = f/22
• 55mm --> min aperture = f/40 (!)

But wait! There's more! It occurred to me that only the maximum aperture is read from the lens via the data pin and that the minimum is calculated from the known range of travel for the aperture actuator on the body. I did a quick series at f/32, f/36, and f/40 (keeping constant shutter speed for all three) and found that the f/36 and f/40 exposures were identical. In short, the aperture actuator was likely not able to run full travel to f/40, but stopped at the very logical f/36.

I don't know whether shooting at f/40 will risk damage to lens or body, though there is probably little sense in intentionally shooting at that setting or even f/36.

Correction: Actually, I do know. The aperture coupling on the lens passively follows the body lever. The minimum aperture on the lens is simply the point at which the coupling lever stops. There is zero risk to lens or body, just the possibility of a small (less than 1/3 stop) overexposure.


Steve

Okay, it seems that this clears things up. Yes, it's a fully automatic lens (aperture/focus). I tested it...set it to 90mm and f40 zoomed back to 28mm and the aperture lowered incrementally to 22. This is quite the revelation. I didn't realize that the smallest aperture number only correlates to the minimum focal length.

So, this raises more questions that I don't understand. I did a "visual inspection" by place the camera in bulb mode, and looking at the front of the lens as a fire the shutter. The results were at 28mm/f22 the aperture created a smaller "hole" or opening, for lack of the proper term, than it did at 90mm/f40...even though f40 is a smaller aperture size than f22. This is really confusing to me, and maybe it's because I don't quite understand how optics really work.

Can anyone explain this?

---------- Post added 11-26-17 at 05:54 PM ----------

I suspect your lens has a variable minimum and maximum aperture that depends on focal length. If the lens is on 90mm you can set it to f/40 but if you zoom to 28mm I'll bet it drops back to f/22. Aperture rings on lenses like that only show the widest numbers that can be achieved and not the effect of longer focal length.

Some lenses have the same min/max aperture at all focal lengths but that is typically more expensive.

---------- Post added 11-26-17 at 03:38 PM ----------

This variability is from the math, the aperture diameter on these variable lenses stays the same at a given setting, but as focal length changes this changed what the f/stop is at that setting. Fixed lenses effectively open wider as the lens zooms to longer focal lengths keeping f stop unchanged.

Yep, you got it. I tried that out, and it worked exactly like you said. So...would the f40 at 90mm really be about the same as f22 and 28mm, in terms of depth of field and light allowed through the lens?

[TheOneAndOnlyJH]

So, this raises more questions that I don't understand. I did a "visual inspection" by place the camera in bulb mode, and looking at the front of the lens as a fire the shutter. The results were at 28mm/f22 the aperture created a smaller "hole" or opening, for lack of the proper term, than it did at 90mm/f40...even though f40 is a smaller aperture size than f22. This is really confusing to me, and maybe it's because I don't quite understand how optics really work.

Can anyone explain this?

There are two reasons for it to look that way, but keep in mind that f number is a ratio, not an exact measurement.

Zooms move their lens elements in complicated ways that might make the aperture look like it's changing size when it is actually staying the same. (Think of how it looks to move a magnifying glass closer and farther from an object.) Even though the physical aperture iris doesn't change the f-number changes because the focal length changed and the iris may have moved to a different part of the optical path.

F number is a ratio of focal length to entrance pupil, so f/2.8 means the focal lengthupil is 1:2.8 (entrance pupil = focal length ÷ f number). People usually just say aperture but the math behind f-number uses the pupil. (The aperture is used to change the effective size of the entrance pupil but it's usually not the same since it depends on where in the lens path it is and how much the light is bent by the lens elements.) This means that at 28mm f/22 the effective entrance pupil is 1.27mm and at 90mm f/40 the pupil is 2.25mm. This entrance pupil is what you're seeing.

Take a look at these two links and it'll explain a bit better than I can:

https://en.m.wikipedia.org/wiki/F-number
https://en.m.wikipedia.org/wiki/Entrance_pupil

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Last edited by TheOneAndOnlyJH; 11-26-2017 at 06:49 PM.

[stevebrot]

Can anyone explain this?

The physical opening stays the same, though the apparent size viewed from the front may vary by zoom setting. Complicated subject...don't worry about it.

So...would the f40 at 90mm really be about the same as f22 and 28mm, in terms of depth of field and light allowed through the lens?

F/40 is f/40 and f/28 is f/28 even though the physical (absolute) aperture size stays the same. F-stop may also be referred to as relative aperture or f-ratio. It is quite literally a ratio where "f" is the focal length and the denominator is the focal length ÷ diameter of the exit entrance pupil (how the aperture looks from the front of the lens).* Since the f-number is relative to the focal length, it means the same in regards to exposure regardless of the focal length. Example:

50mm lens with exit entrance pupil size = 25mm: (50 ÷ 25) ==> f/2
Exit Entrance pupil size for f/2 where f = 50mm: 50 ÷ 2 = 25mm

100mm lens with exit entrance pupil size = 50mm: (100 ÷ 50) ==> f/2
100mm lens with exit entrance pupil size = 25mm: (100 ÷ 24) ==> f/4 (half as much light)

For both lenses the amount of light to the sensor is the same at f/2 and half as much at f/4.

The relationship between DOF, absolute aperture, focal length, focus distance, f-stop, and bokeh is complicated. It is enough to say that what you get is what you get and that f/3.5 (or even f/5.6) at 28mm with your lens will definitely have different DOF than f/5.6 at 90mm and that bokeh will likely be different as well. There is a DOF scale on the topside of your lens opposite the focus scale that provides a good estimate for FF bodies.


Steve

* I should probably have simply said absolute aperture size since the exit entrance pupil size is essentially equivalent for uncomplicated cases.

[stevebrot]

There are two reasons for it to look that way, but keep in mind that f number is a ratio, not an exact measurement.

Thanks for doing a better job explaining than me! I always get my pupils mixed up even thought it is the entrance pupil (obviously) that determines the amount of light.


Steve

[UncleVanya]

So, this raises more questions that I don't understand. I did a "visual inspection" by place the camera in bulb mode, and looking at the front of the lens as a fire the shutter. The results were at 28mm/f22 the aperture created a smaller "hole" or opening, for lack of the proper term, than it did at 90mm/f40...even though f40 is a smaller aperture size than f22. This is really confusing to me, and maybe it's because I don't quite understand how optics really work.

Yep, you got it. I tried that out, and it worked exactly like you said. So...would the f40 at 90mm really be about the same as f22 and 28mm, in terms of depth of field and light allowed through the lens?

So let's take these 1 at a time. First question - many above me answered it better than I will but simply put the way the aperture looks can be deceptive due to the lens elements moving inside of the lens as you zoom. Essentially the opening is probably staying the same.

question 2 - No the f/40 at 90mm is not even close to the f/22 at 28mm. The f/40 really is almost 2 stops slower just like f/4 to f/8. Also at 90mm f/40 will have a lot of depth of field but probably not as much as 28mm at f/22 since wide angle lenses tend to have larger depth of field. The reason the lens aperture can be marked that way is that you are not metering outside the camera most of the time. Typically you are metering through the camera lens and the starting point (lowest number on the lens) for measurement is the wide open setting. It really doesn't matter what this number is - only that as you increase the numbers and restrict the light the ratio of stops is the same. So no matter what the f/22 setting (even at f/40) is the same number of stops from wide open. So the meter will get the result right even if it doesn't accurately describe things (it does since your camera is aware of the variable aperture on this lens.)

[TheOneAndOnlyJH]

Thanks for doing a better job explaining than me! I always get my pupils mixed up even thought it is the entrance pupil (obviously) that determines the amount of light.


Steve

I see you pulled my normal trick of writing such a long reply that by the time you click submit someone else has beat you to it and you end up repeating them.

Don't feel bad about mixing up your pupils. To be honest, I couldn't remember the word 'pupil' at all so I had to check Wikipedia for a quick refresher before I posted.