[Jerry Utah]

I am trying out two used Pentax 645-A lenses (35mm and 45-85mm) with a similar problem---the diaphragm blades have trouble opening all of the way. I use manual aperture control, not A, on the lens's dial. The problem only occurs when the lenses are attached to a camera. The lenses are in mint condition other than this problem.

For example, on the 45-85, I cannot reach f/4.5. The aperture works perfectly opening all the way to f/5.6, but the last step wider is too tight to be able to reach f/4.5 without seeming to risk breaking something. From f/5.6, I can get only about half way to f/4.5.

On the 35mm f/3.5, it is strangely tight to get all of the way open, but its not that bad. Yet I worry that things will soon deteriorate.

Is this an easy repair by myself or using a repair shop? Is it best to just return the lenses for refunds, or perhaps just return the 45-85? Is there something I might be doing wrong? Is mine a common problem?

I do want to shoot both lenses wide open, and I don't want things to get worse. In contrast, I have no problem with my 645-A 300mm f/4, which is my only other 645 lens so far.

[Not a Number]

What body are you using?

When the lenses are not attached can they be switched easily in and out of the "A" setting?

[UncleVanya]

What body are these on? Is an adapter involved?

[Just1MoreDave]

I have an idea about this but disclaimer: I have no experience with the 645 lenses or cameras.

The 45-85 zoom is a constant aperture zoom. These lenses can appear to be slightly stopped down at the wide angle end of the zoom. The aperture blades may be visible when you look through the front of the lens, when the aperture ring is at f4.5. That is completely normal.

I only have a few constant aperture zooms and the only lens that does this is my DA 16-45mm f4. At the 16mm end and wide open, you can see the aperture blades from the front. They aren't in the image frame, though. I believe I first heard about the phenomenon when people were looking at the DA*16-50mm f2.8, wondering if it was really f2.8.

[Jerry Utah]

What body are you using?

When the lenses are not attached can they be switched easily in and out of the "A" setting?

Thanks for your questions.

Yes, easily in and out. Both lenses (35mm and the 45-85mm) easily switch back and forth between "A" and any of the manual aperture numbers f/22 or f/32 to f/5.6. This is true whether the lens is mounted or not.

When either lens is attached, it is attached to either a Fotodiox TLT ROKR adapter or a Fotodiox RhinoCam Vertex adapter---for shifting and possibly tilt---to Sony E-mount. My third adapter from Photodiox is arriving tomorrow (for Graflex 4x5). I will check the lenses on this third adapter too. But I don't yet have a Pentax 645 camera to test my lenses on.

For all three Photodiox adapters (Pentax 645 lens to Sony E camera), I have no use for the "A" setting.

---------- Post added 01-17-23 at 02:57 PM ----------

I have an idea about this but disclaimer: I have no experience with the 645 lenses or cameras.

The 45-85 zoom is a constant aperture zoom. These lenses can appear to be slightly stopped down at the wide angle end of the zoom. The aperture blades may be visible when you look through the front of the lens, when the aperture ring is at f4.5. That is completely normal.

I only have a few constant aperture zooms and the only lens that does this is my DA 16-45mm f4. At the 16mm end and wide open, you can see the aperture blades from the front. They aren't in the image frame, though. I believe I first heard about the phenomenon when people were looking at the DA*16-50mm f2.8, wondering if it was really f2.8.

Bingo! Thanks! I had no idea this behavior of a zoom wide open was possible. Yes, on the zoom, the blades are visible even wide open at the wider focal lengths, and not visible at the more telephoto focal lengths. Exactly as you guessed!

It appears that the best way for me to avoid the excessive pressure on the lens aperture ring that I find needed to get to wide open is to use the lenses a lot more for a while, but more importantly, to set the aperture to the widest manual setting just before mounting the lens.

I will be testing this more later tonight, and I may have some follow-up question tomorrow or later this week. It is close to no problem now, perhaps in part because I've been doing so much checking and loosening things up. And also understanding that the zoom behavior often or usually shows the octagon even wide open.

[tuco]

I don't have any Pentax 645 lenses. But my Pentax 67 lenses have a metal tab sticking out a slot common to the mount that moves with the change in aperture (used for the TTL prism). If that tab's movement was restricted (as in say an adapter not allowing full range of movement), I would also not be able to rotate the aperture ring fully either. Does a 645 lens have such a thing on its lens I wonder?

[Jerry Utah]

I don't have any Pentax 645 lenses. But my Pentax 67 lenses have a metal tab sticking out a slot common to the mount that moves with the change in aperture (used for the TTL prism). If that tab's movement was restricted (as in say an adapter not allowing full range of movement), I would also not be able to rotate the aperture ring fully either. Does a 645 lens have such a thing on its lens I wonder?

Each of my three Pentax 645-A lenses (i.e., earlier 645 lenses, lacking autofocus) have two black tabs in partially-circular slots that look to be outside the visible lens elements. Both of my adapters have two corresponding silver-colored metal tabs, perpendicular to the black metal tabs of the lens. Using either of the adapters, I can easily see what's going on with the two black and the two silver tabs. I can see quite clearly that one of the two black tabs opens the aperture blades, and the other has to be engaged as well to override full-aperture metering. That's my inference, anyway. My adapters engage both so that changes to the aperture setting immediately open or close the lens aperture. My adapters don't meter light.

Both of my adapters now seem to be working well in almost all regards. Most of the tightness that I was complaining about earlier is now gone in both lenses. And often, the tightness is now completely gone. I appears that teim spent varying the apertures in manual mode has loosened the mechanisms so that they work better. My trick of setting the aperture to the widest manual setting on the lens before attaching the lens to the adapter seems to help a lo. Or, it is possible that somehow, without knowing what habit I've changed, my current procedure of attaching the lens set wide open in manual mode and and varying the f/stop afterwards is somehow working well now for reasons that I can't guess.

I don't pretend to know everything about what's going on or what the future holds for the aperture blades of these two lenses, so further insights like these from tuco and everyone else are still appreciated.