[Shawn67]

Curious if the late Pentax 67 versions of the 45mm and 55mm have adjustable infinity stops? Both of mine focus beyond infinity, the 45mm more than the 55mm. My SMC 6x7 105mm is dead on at infinity.

Focus was tested directly with a second camera focused at infinity looking through the Pentax with ground glass on the 6x7 film gate with Xs on it. This also confirmed the focus screen was set properly.

Thanks!

[kaseki]

I can't answer the question, but suggest that when measuring you do so with the camera and lens at each of the temperature limits that you may expect them to endure when taking photographs.

[itsdoable]

The infinity/focus adjustment on these legacy lenses are done with the setscrews that are on the focus barrel, under the rubber grip (if present).

[TDvN57]

My view on infinity focus: it seems to be a relevant term, depending on who you speak with. Some regard it as focusing to +/-100m, others have a different opinion. With longer focal lengths some say infinity is if you can focus on the moon.

My preference is that a lens should focus well beyond whomever's definition of infinity. I have found differences in focusing on the moon versus stars or Andromeda for example.

A lens that locks at some definition of infinity is guaranteed to disappoint someone at some point trying to focus at a distant object. Then the lens will be blamed as not being sharp at distance, better to stop it down. It might be good advice to stop it down to band-aid a situation, however if the lens was able to focus beyond infinity, the lens will most likely remain sharp at a distance.

My advice: if the focus ring turns beyond infinity, count your blessings.

[Shawn67]

The infinity/focus adjustment on these legacy lenses are done with the setscrews that are on the focus barrel, under the rubber grip (if present).

Thanks, I will check that out and see if these have that or not.

---------- Post added 04-02-23 at 08:36 AM ----------

My view on infinity focus: it seems to be a relevant term, depending on who you speak with. Some regard it as focusing to +/-100m, others have a different opinion. With longer focal lengths some say infinity is if you can focus on the moon.

My preference is that a lens should focus well beyond whomever's definition of infinity. I have found differences in focusing on the moon versus stars or Andromeda for example.

A lens that locks at some definition of infinity is guaranteed to disappoint someone at some point trying to focus at a distant object. Then the lens will be blamed as not being sharp at distance, better to stop it down. It might be good advice to stop it down to band-aid a situation, however if the lens was able to focus beyond infinity, the lens will most likely remain sharp at a distance.

My advice: if the focus ring turns beyond infinity, count your blessings.

My definition of infinity is using a second camera to check infinity focus, like a collimator. When done this way actual infinity is very sharp.

In my experience a lens that focuses beyond infinity tends to not be as sharp as it should be when trying to focus at infinity as the viewfinder isn't as precise as the collimator and a hard stop. Hence wanting to adjust my wider angles to stop at actual infinity focus. A lens focused beyond infinity is not sharp anywhere and stopping it down is the band-aid for missed focus. The 45 is badly out of focus at the stop and wide open looks soft everywhere, the 55 is less so, my 105 is dead on at the stop.

[TDvN57]

Thanks, I will check that out and see if these have that or not.

---------- Post added 04-02-23 at 08:36 AM ----------



My definition of infinity is using a second camera to check infinity focus, like a collimator. When done this way actual infinity is very sharp.

In my experience a lens that focuses beyond infinity tends to not be as sharp as it should be when trying to focus at infinity as the viewfinder isn't as precise as the collimator and a hard stop. Hence wanting to adjust my wider angles to stop at actual infinity focus. A lens focused beyond infinity is not sharp anywhere and stopping it down is the band-aid for missed focus. The 45 is badly out of focus at the stop and wide open looks soft everywhere, the 55 is less so, my 105 is dead on at the stop.

I think you misunderstand what I'm saying... Anyway have fun.

[itsdoable]

Some lenses focus beyond infinity by design to accommodate environmental factors -typically longer focal lengths and those with internal focus. Shorter unit focus lenses usually have a hard infinity stop because environmental variations are within the lens' DoF. Having the lens focus past infinity means you will always be able to reach it. Having the stop calibrated for the environmental conditions you plan to use is useful for things like astro or aerial, where manual focus can be tricky.


That said, if the lenses do not have any signs of being manipulated, I would check everything else before adjusting the infinity stop. Loose element, loose mount, loose helicoid, etc...

[TDvN57]

If you want to check of your lens can really focus to infinity, within human limits, then use a Bahtinov filter and focus on the stars, with the lens aperture wide open. I think Nisi makes an advanced version as a filter option. Otherwise a Bahtinov will cost you about $20 and fits a range of lens sizes.

[kaseki]

I have found differences in focusing on the moon versus stars or Andromeda for example.

If my math[s] is correct, for an ideal lens the difference at the focal plane between true infinity and some distant object such a the moon approximately equals focal length squared divided by distance to the object (moon). For a 1000-mm lens and 3.84 x 10^11 mm moon this is 2.6 nm, three orders of magnitude below the pixel size. This suggests that the difference in focus setting is driven not by definition of infinity at astronomical distances, but by best focus among a variety of aberrations of real lens assemblies affecting different sized objects and/or different aperture settings. (I think this is another good reason for having a bit of infinity focus overshoot.)

(If you operate an observatory with telescope focal length >> a meter, such as the ~17m focal length Hale telescope, best focus for micron sized pixels or photographic plates could indeed vary slightly between Andromeda and the moon.)

[TDvN57]

If my math[s] is correct, for an ideal lens the difference at the focal plane between true infinity and some distant object such a the moon approximately equals focal length squared divided by distance to the object (moon). For a 1000-mm lens and 3.84 x 10^11 mm moon this is 2.6 nm, three orders of magnitude below the pixel size. This suggests that the difference in focus setting is driven not by definition of infinity at astronomical distances, but by best focus among a variety of aberrations of real lens assemblies affecting different sized objects and/or different aperture settings. (I think this is another good reason for having a bit of infinity focus overshoot.)

(If you operate an observatory with telescope focal length >> a meter, such as the ~17m focal length Hale telescope, best focus for micron sized pixels or photographic plates could indeed vary slightly between Andromeda and the moon.)

Thank you for the explanation. I observed a difference with the 645 600mm + 1.4TC. Was a light touch that made a difference in sharpness of focus. I found it strange, but your explanation demystify my observation. With the larger lenses the temperature change of the lens body will also mess with a "fixed" focus setting.

One of my favorite 67 lenses mounted on the 645z will focus fantastic up to about 1km. Beyond that the focus ring stops at infinity but it is still out of focus. I thought the lens was faulty and I got another one, but it had the same problem. I "shaved" off about one mm from the 67-645 adapter (Pentax brand adapter) and now I can focus far distant objects (tested up to about 10km with overrun left). Haven't tried the moon yet because I'll most likely never use this lens for that.

[Shawn67]

Tried checking under the rubber focus ring on the SMC Pentax 67 F4 45mm and there are no adjustment screws for the infinity stop so that must be done in a different manor on this lens.

To give a sense of what I am talking about.



This is at 100% zoom in with the image on the right focused at 'infinity' on the lens, left is focused like this:



This position is just about the same as what I found on the Pentax 6x7 optically using another camera as a collimator. The suggests the adapter on the 645D is the correct thickness.

A 45mm at f4 on the 645D focused at infinity should have everything from about 44' and on in focus. At 'infinity' on the lens nothing is in focus.

[TDvN57]

Tried checking under the rubber focus ring on the SMC Pentax 67 F4 45mm and there are no adjustment screws for the infinity stop so that must be done in a different manor on this lens.

To give a sense of what I am talking about.



This is at 100% zoom in with the image on the right focused at 'infinity' on the lens, left is focused like this:



This position is just about the same as what I found on the Pentax 6x7 optically using another camera as a collimator. The suggests the adapter on the 645D is the correct thickness.

A 45mm at f4 on the 645D focused at infinity should have everything from about 44' and on in focus. At 'infinity' on the lens nothing is in focus.

Pse let me know if I understand correctly:
1) The lens focus correctly? Near objects as well as far objects can be focused correctly?
2) The distance scale on the lens does not align with the actual distance?
3) when the focus ring is turned towards the infinity mark then the lens is focused beyond infinity, which is overrun?

If I am correct with the above I would suggest to use the lens as is. In my experience I have never found a lens with an accurate distance scale, unless it is a high end film/video lens where focus is set by measuring the distance from camera to the subject with a tape and dial in the lens focus on the distance scale accordingly.

In photography I have never seen that used as a means of focus, mainly because the scales on the lenses are too compressed to read accurately, focus is rather done visually, either by a human or camera, still visually. The only practical use I have found for the scales are to get an estimate of the depth of field. But since we can do a visual check of the depth of field since the 80's and before, which is in any way more accurate if checked visually, I don't see much practical use of the distance scales.

Hope this helps. I would still recommend to get a Bahtinov filter/screen, then you can do the same exercise on a bright star, or the moon and you may see a different result on the distance scale.

Hope this helps.

[Shawn67]

For your questions...

1) Yes, the lens focuses correctly.
2) Correct, the focus distance on the lens is not accurate, neither is the infinity stop. It is really just the infinity stop I am trying to get proper. Trying to eyeball it in the viewfinder isn't going to be as sharp as if the stop were just correct in the first place. Esp. in lower light.
3) Yes, if focused to the infinity stop (also the infinity mark) the lens is out of focus.

I'm sorry you haven't found a lens with a correct infinity stop/focus distance. That is kind of shocking to me as I've got dozens of them that are accurate. Including many Pentax lenses. This one isn't, neither is the 55mm, though it isn't off as badly. Pretty much any rangefinder lens is going to need to have an accurate infinity stop. The cameras rangefinder is literally calibrated against the lens at the stop.

Also, very surprised that you haven't seen it as a means of focus. Scale focusing is common, there are many cameras cameras that can *only* be focused that way. Try shooting a Rollei 35 for example. I've built many panoramic cameras that are scale focused and having infinity properly focused at the stop is crucial for getting sharp images out of them. Scale focusing is also a common practice with wider angle lenses, such as a 45mm on a 6x7 camera.

I won't see a difference against a star or moon. Again, I've tested this optically using a collimator setup. If it were actually focusing at infinity at the infinity stop I'd be in focus from about 45 feet away and on. Nothing is in focus even hundreds of feet away or through the collimator setup. Because the lens is out of focus at the infinity stop. This is a 45mm lens, not a 1000+ mm rig.

In the viewfinder the difference from infinity stop to actual in focus at infinity is pretty subtle and a little difficult to see on a tripod. Handheld it is harder still, low light even more difficult. Being able to just set the lens to the infinity stop will result in sharper pictures when I am shooting infinity. That is why I'd like to adjust the lens so infinity is focused at the infinity stop.

[Shawn67]

Shooting the moon with a 45mm on the 645D....



Zoomed *way* in... Image on the left is at the infinity stop, image on the right is focused in a little bit. Not focused properly but clearly more in focus with more contrast and less CA.



As a comparison this is the SMC 67 200mm F4 at the infinity stop (left) vs focused in slightly (right).

[itsdoable]

You are testing infinity of a 67 lens on the 645 with an adapter - I though you were testing on the P67 from your original post.


Adapters are usually made marginally shorter in the tolerance, so you can reach infinity, and this is more noticeable on wide angle lenses, although I'd expect you to see this difference on your 105 as well. But adapters add another variable.


I would test it on a P67 first, and calibrate it there. But if you don't care about compatibility and use on other bodies, you can just adjust all your lenses to fit your current equipment. I don't have the documentation for the 4/45, and I've not had to adjust mine, but previous 67 lenses have 3 tiny set screws in the focus barrel, under the rubber ring where present. Some lenses have the focus adjust under the beauty ring. but I've not experienced that with Pentax.