[unlqing]

lol, just got a nice F*300/4.5, great lens. But it would be perfect if the seller didn't throw the tripod collar away~~

[Ben_Edict]

I think we didn't appreciate just how hyper critical mechanical and optical factors can get when dealing with a 400mm F/2.8.

That's very true. I know the theory, but seeing this in a practical example is a different thing. I really did not think the effect of a missing filter to be that prominent, rendering the lens useless…

Ben

[Ben_Edict]

This has indeed been a incredibly helpful discussion: In addition to solving Lee's problem it's been a graduate seminar in long lens technique. I have two follow-up questions though:

1. My lens doesn't have a filter either and, although it's taken me a while to get the kind of results I'm looking for, I'm getting there--even using the lens with a TC on a monopod. I certainly welcome the possibility of increased resolution but why has the lack of a filter has not been as significant issue for me as it has been for Lee?

Here's a (filterless) A*400 + F 1.7x + tripod shot from yesterday.

2. Lee seems to have solved his problem with a "garden variety" front-element filter so it seems acquiring an appropriate filter is not as difficult as I imagined. I gather that a drop-in is not a completely different animal and, although a pentax smc may be preferred, the Hoya hmc I borrowed from my 43mm ltd might do "ok". Is that right?

First the effect will be pronounced with a fast lens. Which one do you use? The f/2.8 or the f/5.6, though I have the latter and don't think it has a rear filter slot at all.

Secondly: the brand of the filter is not important. The thickness of the glass makes the difference in shifting the focal plain and lens correction. I don't think 0.1mm is critical, but there would be a difference between a 1mm thin and a 2mm thick glass.

Ben

[dadipentak]

First the effect will be pronounced with a fast lens. Which one do you use? The f/2.8 or the f/5.6, though I have the latter and don't think it has a rear filter slot at all.

Secondly: the brand of the filter is not important. The thickness of the glass makes the difference in shifting the focal plain and lens correction. I don't think 0.1mm is critical, but there would be a difference between a 1mm thin and a 2mm thick glass.

Ben

Thanks for the response, Ben. I have A*400 f/2.8, theoretically just like Lee's. I get you're point about thickness and I'll pay attention to that but I was under the impression that a drop-in was very different from the front-end filters I'm used to and I now understand that's not the case.

[Ron Boggs]

So great to see Lee shooting sharp shots...YEAH!

Regarding filter choices...Ben points out the glass thickness issue, to which I will add the glass placement issue. Some filters hold the glass closer to the front of the filter ring, others closer to the back or male threaded end. Just as glass thickness can effect the optical formula, so too can the front or back mounting placement of the glass within the rings. That's why I recommend the Pentax SMC filters as they are presumed to be tested to work.

We may be overthinking this issue however, based on the discussions above. And I must admit, when I had the A*300/2.8, I purchased a few B+W filters for it. Those standard filters had such thick rings that the drop-in drawer wouldn't drop-in with the B+W filters in place. No problem...I just filed the filter rings down so they would fit. Note that this didn't affect glass thickness nor placement. Nowadays, many filter manufacturers offer slim versions of their filters for use with vignette-prone wide angles. If anyone is going to try B+W filters for the drop-in, I recommend the slim models so you don't have to use a file like I did.

The FA* lenses with drop-in filter holders came with a filter kit so you can be assured that the filters are correct for that lens. The A* lenses did not ship with filters included, thus presuming you would simply screw in a Pentax filter. From the current discussion, there may be enough tolerance regarding precise glass placement that the precise filter may be less important than simply having those surfaces (front and back of the filter glass) somewhere in the drop-in area of the light path as it goes through the lens?

Regardless, the lesson here is if your lens has a drop-in filter drawer, you should probably keep a filter in it!

[Ben_Edict]

Thanks for the response, Ben. I have A*400 f/2.8, theoretically just like Lee's. I get you're point about thickness and I'll pay attention to that but I was under the impression that a drop-in was very different from the front-end filters I'm used to and I now understand that's not the case.

The glass is not different. The filter mount may be slimmer for drop-in filters. At least in my Tamron 300/2.8 not all filters fit through the slot.

Ben

[Ron Boggs]

Oh, and why would one 400/2.8 require a drop-in filter to focus and another copy of the same lens not need a filter to focus sharply?

One possible answer would be temperature. All the * teles have focus rings that can focus past infinity and Pentax lens literature says that's to compensate for temperature fluctuations which change the focus point slightly. Perhaps the two 400's being discussed in this thread are being used in vary different temperatures? Perhaps one is at room temp and the other at field temps?

[gofour3]

Here is a link to the original Pentax manual for these A* series telephoto lenses:

http://www.pentax.ca/pdf/o_manuals/om_A300MM400MM600MM1200MM_e.pdf


and one for the older K series telephotos:

http://www.pentax.ca/pdf/o_manuals/om_SMC500MM1000MMREFLEX_e.pdf

Phil.

[heliphoto]

...

Also, the plano-parralel filter at the rear of the lens will offset the focal plain, increasing the backfocal distnace, as I wrote above. I guess, that especially in these fast lenses, the backfocal distance is so exacting, that the 2mms which the filter adds decide over in-focus or out-of-focus, without a remedy. With slower lenses (think, a 400/5.6), this would not be that significant.
But I confess, I am myself quite surprised, that the 400/2.8 does not allow for more focus "beyond infinity", which would have allowed to copmpensate for the lack of the filter (but ofcourse not for its slight correction of field curvature.

We should somehow make an essay out of this thread, there is a lot info worth to be preserved.

Ben

Now it's my turn to think out loud...

Does the A*400/2.8 incorporate a "Floating" lens element or group? These are supposed to compensate for aberration differences at different focus distances...

My theory is that the absence of a filter offset the focus point (decreased the backfocal distance as described by others) and caused Lee to have to correct focus by turning the focus ring and moving the elements, but if a floating element or group is used, this new position would incorrectly correct for aberration, since it would be improperly positioned with regard to the actual focusing distance. This doesn't explain why dadipentak is able to get good results without a rear filter however ... Also note that I'm way out of my depth here and have no idea what I'm talking about ...

[Ben_Edict]

That was just a cross-post with my own short answer…

So great to see Lee shooting sharp shots...YEAH!

Regarding filter choices...Ben points out the glass thickness issue, to which I will add the glass placement issue. Some filters hold the glass closer to the front of the filter ring, others closer to the back or male threaded end. Just as glass thickness can effect the optical formula, so too can the front or back mounting placement of the glass within the rings. That's why I recommend the Pentax SMC filters as they are presumed to be tested to work.

If Pentax did not provide a filter for the filter holder, I must confess, I would expect the lens to work without one. At least they should have included info on exactly which filter would be be required for any specific lens.

We may be overthinking this issue however, based on the discussions above. And I must admit, when I had the A*300/2.8, I purchased a few B+W filters for it. Those standard filters had such thick rings that the drop-in drawer wouldn't drop-in with the B+W filters in place. No problem...I just filed the filter rings down so they would fit. Note that this didn't affect glass thickness nor placement. Nowadays, many filter manufacturers offer slim versions of their filters for use with vignette-prone wide angles. If anyone is going to try B+W filters for the drop-in, I recommend the slim models so you don't have to use a file like I did.

Yes, the dimensions of the filter mount can be a problem, as I experienced myself, when searching for a drop-in filter for my Tammy 300/2.8. Some did not fit into the narrow slot. Filing down is a simple solution and works nicely.

The FA* lenses with drop-in filter holders came with a filter kit so you can be assured that the filters are correct for that lens. The A* lenses did not ship with filters included, thus presuming you would simply screw in a Pentax filter. From the current discussion, there may be enough tolerance regarding precise glass placement that the precise filter may be less important than simply having those surfaces (front and back of the filter glass) somewhere in the drop-in area of the light path as it goes through the lens?

Positioning of the filter in the light path should, as you rightly pointed out, not be obverlooked. But I think, with these long focal lengthes, we are talking about, the slight misplacement due to different filter mounts should be within acceptable margins of error. Glass thickness is much more important. It would be different for wide angle lenses, though, or basically any lens with a less parallel bundle of rays, unless its construction is near telecentric.

Regardless, the lesson here is if your lens has a drop-in filter drawer, you should probably keep a filter in it!

Hm, I am not convinced this is an automatism. For instance there has been the first Tamron 300/2.8 (107B) perfoms better without the filter in place, because the filter seems to introduce spherical abberation. Tamron engineers designed the lens, without the filter in place. I also think, the old Pentax K 500/4.5 was designed withou the rear filter (filter changing on that lens is a nightmare). I would think, sometimes they simply added a slot (or a gelatine holder in the case of wide angle lenses), to allow the use of smaller, cheaper filters and did obviously not take into account the function as an additional refreactive element in the light path.

Ben

[Ben_Edict]

Now it's my turn to think out loud...

Does the A*400/2.8 incorporate a "Floating" lens element or group? These are supposed to compensate for aberration differences at different focus distances...

My theory is that the absence of a filter offset the focus point (decreased the backfocal distance as described by others) and caused Lee to have to correct focus by turning the focus ring and moving the elements, but if a floating element or group is used, this new position would incorrectly correct for aberration, since it would be improperly positioned with regard to the actual focusing distance. This doesn't explain why dadipentak is able to get good results without a rear filter however ... Also note that I'm way out of my depth here and have no idea what I'm talking about ...

As far as I can see from Dimitrov's wonderful site, there is no floating element in that particular lens. But it sports internal focusing, which could have an effect similar to the one you supposed. Why dadipentak's lens works nicely without the filter in place is a mystery to me, then …

Ben

[dadipentak]

Also note that I'm way out of my depth here and have no idea what I'm talking about ...

ditto!

I have to turn my attention to other things for a while but:
1. I note that the manual says little or nothing about filter specs.
2. I did a little testing with the Hoya filter in place. I'd say if anything, my results were worse than w/o filter but lighting conditions were less than ideal. It may also relate to filter thickness. I'll do more testing later in the week.

[imtheguy]

ditto!

I have to turn my attention to other things for a while but:

What the hell could be more important than this kind of fun? Nevermind, don't answer.

For what its worth, the freebee 49mm I just happened to drop in is an old looking B+W, UV+Haze, F-PRO

I know nothing about B+W filters, in fact I thought it was a skylight filter until I just read the inscription.

[Ben_Edict]

What the hell could be more important than this kind of fun? Nevermind, don't answer.

For what its worth, the freebee 49mm I just happened to drop in is an old looking B+W, UV+Haze, F-PRO

I know nothing about B+W filters, in fact I thought it was a skylight filter until I just read the inscription.

B+W is a division (now) of Schneider-Kreuznach. They use mainly Schott glass and are among the two or three most highly respected manufacturers of filters. Some of their filters are also sold by Zeiss. (Other first tier names are Heliopan, and ofcourse the top modells of Hoya - but Hoya also makes some cheap crap.)

Ben

[wildman]

What the hell could be more important than this kind of fun? Nevermind, don't answer.

For what its worth, the freebee 49mm I just happened to drop in is an old looking B+W, UV+Haze, F-PRO

I know nothing about B+W filters, in fact I thought it was a skylight filter until I just read the inscription.

Which further indicates that the filter is a critical active (rather than passive) optical element in the over-all design of the lens.

It doesn't matter as much what kind of filter but you had better come pretty close to the refractive qualities that Pentax had in mind when they designed the lens which, I presume, is primarily a function of thickness all else being equal.

Given the low refractive index of a very thin flat piece of filter glass who would have thought it could have had such a profound effect on focus.