Metering and exposure settings with mirror lens

[328davemc]

Hi. For grins I purchased a used Sigma 600mm f8 mirror lens. OK, I know most of you probably aren't fans of mirror lens, but we'll save that discussion for another post.

I knew this was a manual lens -- fixed f8 aperture, no autofocus. It didn't occur to me that when I mounted the lens on my Nikon D7500 the camera didn't recognize the f8 aperture. Of course, I realized, the camera has no electronic connection to the lens. But being optimistic (or having not thought all this through completely) I hoped I could somehow use the camera's TTL metering to to find the correct shutter speed for proper exposure. Nope. The camera will not fire unless in Manual mode, and never shows an exposure scale in the viewfinder.

So, what are my choices? Am I doing something wrong that can be fixed via a camera setting so that I can get metering from my camera? Do I need a separate light meter? By setting the ISO high and shooting on a tripod at 1/1000 second in good light I'm getting ok photos, but I still I'd like to know precisely what limitiations my camera/lens setup has.

Please, any advice or recommendations would be greatly appreciated, but kindly refrain from telling me the mirror lens is only good as a paper weight! Thanks.

 

[Hardus Nameous108]

I use SLR lenses on my D3200 in manual mode quite a bit. I just fire off a test shot or two and look at the histogram.

 

[greybeard26]

What happens if you shoot MANUAL with AUTO ISO?

 

[wfooshee]

The D7500 does not have the meter coupling tab on the lens mount, shown here on a D7100, and without that, the camera cannot read the mechanical aperture setting. Even though it's a fixed-aperture lens, I believe it had a tab that engages that coupling tab as the lens is mounted, so it can communicate its f:8 status to the camera. That's why I never bought a D7500, as I have several legacy AI non-AF lenses. They meter just fine on my D7200 and D500, but will not meter on a D7500.

The without the mechanical link provided by the AI coupling tab, or a digital link to the lens, the camera has no idea what the aperture is. You'll have to shoot manual settings and specify f:8.

 

[328davemc]

The D7500 does not have the meter coupling tab on the lens mount, shown here on a D7100, and without that, the camera cannot read the mechanical aperture setting. Even though it's a fixed-aperture lens, I believe it had a tab that engages that coupling tab as the lens is mounted, so it can communicate its f:8 status to the camera. That's why I never bought a D7500, as I have several legacy AI non-AF lenses. They meter just fine on my D7200 and D500, but will not meter on a D7500.

The without the mechanical link provided by the AI coupling tab, or a digital link to the lens, the camera has no idea what the aperture is. You'll have to shoot manual settings and specify f:8.

That's interesting -- my 7500 does not have that tab, but it does have a tab that looks like that, but is on the opposite side of the opening (at about 7 o'clock vs. 1 o'clock).

I thought if I could indeed specify f8 then my camera's metering would pick the correct shutter. But with this lens, aperture is NOT displayed in the viewfinder and the aperture thumb wheel is inactive. So there's no way to tell the camera I have an f8 lens on it. That's what makes me think my only accurate option is to use a light meter. Thanks!!

 

[JBPhotog]

The D7500 is not compatible with non-CPU lenses for metering since the camera has no idea what aperture the lens is set to. Only fully manual M mode will function and using the histogram will get you there. Other cameras that do have the AI tab and can have custom non-CPU settings, allow you to assign focal length and aperture as a custom setting. Once this is done, simply mount the lens, go into he custom lens menu and choose the saved settings, the camera then knows the f-stop and focal length which gets embedded in the EXIF data, you choose the other 2 parameters to affect exposure.

 

[snowbear107]

There are a couple light meter apps for smart phones that seem to work, though I've never really used them. An external meter can be a good thing, either new or used.

 

[petrochemist108]

Most of my cameras meter just fine in aperture priority, with my mirror lenses & other manual lenses.
I think the only exception is my Nikon camera (a J1) I find the use of this camera very much diminished by by it's determination to cripple it (turning off metering & IIRC focus assist) when the camera can't talk to the lens.

No such issues on any of my Pentax or Sony bodies.

Though the IQ from mirror lenses doesn't reach the levels refractive lenses often achieve their light weight for a given focal length can sill be a significant advantage.
One of my cheaper 500mm mirror lenses is light enough to use with the Techart pro to get AF on my A7ii sadly none of the other mirror lenses I have can safely do that.
The MTO 1000mm f/11 is heavy enough that I won't even try to hand hold it, which has also proved quite impractical on any of the bodies with crop factors above 2.

 

[wfooshee]

That's interesting -- my 7500 does not have that tab, but it does have a tab that looks like that, but is on the opposite side of the opening (at about 7 o'clock vs. 1 o'clock).

I thought if I could indeed specify f8 then my camera's metering would pick the correct shutter. But with this lens, aperture is NOT displayed in the viewfinder and the aperture thumb wheel is inactive. So there's no way to tell the camera I have an f8 lens on it. That's what makes me think my only accurate option is to use a light meter. Thanks!!

That tab at 7:00 0'clock is the focus drive for early AF lenses. The earliest Nikon AF lenses did not have motors in the lens, they were driven by the camera, called a screw drive. The D3X00 and D5X00 lines do not have the focus motor in the camera so they require lenses with built-in motors for auto-focus.

As for the tab you don't have, the camera will not meter with that lens, or with any non-CPU lens, because it can't read the aperture from the lens. You have to shoot in manual mode, and specify shutter and ISO and just know that you're exposing at f:8, but without a CPU in the lens or that meter coupling tab on the camera, the camera has no way to know anything about the aperture setting, and there's no way to even inform the camera about the aperture.

As to why the tab is called a meter coupling, a brief explanation... Here's a picture of the tab my D7200 in it's normal position, no lens mounted. For the second picture I moved the tab around the lens mount and I'm holding the ring with my thumb, just to show that the tab moves. Its position around the lens mount is what tells the camera what the aperture setting is for a non-CPU lens; the further around from the starting point, the smaller the aperture. Modern Nikon lenses (G- and P-series lenses) do not have an aperture ring because with the CPU in place, and the electronic controls in the camera, there's no need for it. (That does limit what legacy bodies those lenses work on, though.)

Nikon lenses without an aperture ring do not engage this meter coupling, as the CPU communicates all the necessary information to the camera. Those lenses work on any of Nikon's digital SLRs with the exception of the P lenses not being able to focus on some of the older digitals. There are several legacy film cameras that are unable to use G or P lenses at all.

Any Nikon camera will meter properly with any Nikon-mount lens that has an aperture ring, except those cameras that do not have the meter coupling tab (like yours.) A lens with an aperture ring has a "shelf" on the aperture ring that engages that tab, and as the aperture ring is rotated, it pushes the tab around, which is how the camera reads the aperture setting from the lens. Here's a shot of one of my lenses that has the aperture ring. (This describes Nikon's AI lenses, which came out in 1977. Pre-AI lenses had "bunny-ears" that engaged a pin on the camera body to couple the aperture ring to the camera, and with those lenses you had to "train" the camera for the maximum and minimum aperture of the lens every time you mounted it, by turning the aperture ring to the largest aperture and then the smallest.)

My point here is not really to help you get around the limitation of your lens, but to explain what the tab does, and why it's even called the meter coupling ring. As others and myself have said, you'll have to use manual settings on the camera and a handheld light meter, as the camera is incapable of metering with any non-CPU lens attached. You might be able to use stopped-down metering with that lens. To try it, press the depth-of-field preview button (you might have to select which button does this in the menus) and read the exposure meter in the viewfinder, adjust the shutter speed or aperture, take another reading with the depth-of-field preview, until you get the exposure display centered. You can't use matrix metering mode for this, you have to select center-weighted or spot, and there's no way to adapt this for any auto-exposure; you're still using fully manual settings on the camera, but the meter should give you an accurate reading in stopped-down mode.

 

[petrochemist108]

As for the tab you don't have, the camera will not meter with that lens, or with any non-CPU lens, because it can't read the aperture from the lens. You have to shoot in manual mode, and specify shutter and ISO and just know that you're exposing at f:8, but without a CPU in the lens or that meter coupling tab on the camera, the camera has no way to know anything about the aperture setting, and there's no way to even inform the camera about the aperture. <snip>
As others and myself have said, you'll have to use manual settings on the camera and a handheld light meter, as the camera is incapable of metering with any non-CPU lens attached. You might be able to use stopped-down metering with that lens. To try it, press the depth-of-field preview button (you might have to select which button does this in the menus) and read the exposure meter in the viewfinder, adjust the shutter speed or aperture, take another reading with the depth-of-field preview, until you get the exposure display centered. You can't use matrix metering mode for this, you have to select center-weighted or spot, and there's no way to adapt this for any auto-exposure; you're still using fully manual settings on the camera, but the meter should give you an accurate reading in stopped-down mode.

Other manufacturers have realized that stop down metering works perfectly well without the camera (or the photographer) knowing what the actual aperture is. AFAIK Nikon is the only company that refuses to let the internal metering work as normally as possible.
If the camera is left to meter wide open then stopped down for shooting the exposure will be dark unless the photographer compensates I've fallen foul of this occasionally with lenses that auto stop down on taking an image but don't talk to the camera. In many cases the resultant image is still good enough after processing for record keeping/snap shot puposes.

 

[wfooshee]

Stopped-down metering does work on Nikons, I just don't know for sure if it works on his particular camera/lens combination. I have a pre-AI 50mm lens that I can only mount on my F4 (because its AI coupling tab can be flipped up out of the way.) It won't use any AE mode on the camera, but I can use stopped-down metering.

As for "refuses top let the internal metering work as normally as possible," the Canon-mount version of that lens will also not auto-expose and the Canon camera must use stopped-down metering or a handheld light meter to calculate exposure.

 

[petrochemist108]

Stopped-down metering does work on Nikons, I just don't know for sure if it works on his particular camera/lens combination. I have a pre-AI 50mm lens that I can only mount on my F4 (because its AI coupling tab can be flipped up out of the way.) It won't use any AE mode on the camera, but I can use stopped-down metering.

As for "refuses top let the internal metering work as normally as possible," the Canon-mount version of that lens will also not auto-expose and the Canon camera must use stopped-down metering or a handheld light meter to calculate exposure.

I've not got much experience with Canon, but I thought they allowed aperture priority without lens communication (provided shoot without lens is enabled in the menu). Certainly all of my Pentax, Panasonic & Sony bodies do, I think my Sigma does as well but I'm not sure I've used it with adapted lenses to confirm this.
I'm glad to here there re some Nikon bodies that still allow the meter to operate, it seems with my J1 that's only possible if using a chipped adapter, and those chips are now hard to get hold of.