XPAN - when to use/when not to use the center filter?

[reisenberger]

What's people's experience with when/when not to use the 49mm centre filter on

the Hassy XPAN 45mm and 90mm lenses? (with slide film)

 

 

On the 45mm lens, discussions seem to agree it's essential at f4-f8, but what

are people's experiences at f11-f22? Hasselblad say it's not necessary, but I

read that many people leave the centre filter on the 45mm even at f11-f22 -

what's the reason for leaving the centre filter on then? Are people still

finding there's some residual vignetting which the centre filter corrects? (Or

is it just that it's easier not to keep changing the centre filter on and off?)

 

 

For the 90mm lens, I read that most people don't use the centre filter 'at

all'. So are people happy shooting the 90mm lens even at f4 without the centre

filter?

 

 

Many thanks. Background: I shoot exclusively Fuji slide film. Most of my work

is tripod-based landscapes shot around f16 for depth-of-field, hence

particularly the question about filter or no-filter on the 45mm at f16...

 

(Have just purchased the XPAN kit plus 45mm/90mm lenses and centre filter -not

30mm lens yet:)..., after being able to try a second-hand one on a hire basis,

but wasn't able to try the center filter from the hire outfit.)

[josphy]

I have no personal experience with this myself, but I have read Lee Frost's book about panoramic photography, and he uses an Xpan, and he says he never uses the center filter on the 45mm. He doesn't mention specific apertures, but I'm guessing usually stopped down fairly well for landscapes. I would suggest you just test and see if you will be happy with the results. All it will cost you is one roll of slide film -- not much $ compared to an Xpan.

[robert_ardinger]

Based on my use of my XpanII and the 45mm and 90mm lenses:

 

On the 90mm it is not needed at all.

 

On the 45mm there will be noticable light falloff at wide apertures. This is not always bad, just present and the degree it is noticable depends on subject matter.

 

On printing B&W negative (and I suspect color negative) with a conventional enlarger I do not find the light fall off all that noticable but when I scan the negatives I can see it more easily (due to the optical differences of printing with an optical system containing a lens that projects an images and has light fall off of it's own and a scanner that simply records what is on the negative).

 

I do not shoot slide film but since the image is a positive I would think the effect would be worse if printing with an enlarger and essentially the same as negative film if scanning.

 

At small apertures with the 45mm (11, 16) I do not think I can see moch ill effect of not using it.

 

The light fall off it fairly easy to correct for and many times duplicates the "corner burning" I might do anyway BUT if you want evenly light images with the 45mm at wide apertures you will want to use it.

[jeff.grant]

I don't own a center filter and have never had any issues. Having said that, I will always use f/16 or above. My recollection is that Hasselblad said that it wasn't neeeded above f/8.

[db1]

Hasselblad says the filter is ONLY NEEDED when using positive film and shooting at f4 - f8.

[ilkka]

90 does not need a centre filter. In my 45 I have the centre filter always in place. I would only take it off if I am handholding the camera in dim light and really need to get one stop faster speed (very rarely). 30 also needs centre filter all the time.

 

Some people happily use the 45 without CF. I tried it for couple of weeks and then bit the bullet and bought the (expensive) centre filter.

[ilkka]

Where does Hasselblad say that "the filter is ONLY NEEDED when using positive film and shooting at f4 - f8."

 

This is from Hasselblad website where they talk about the CF filter for 45mm lens:

 

For even illumination at all apertures. Recommended for critical applications using transparency film and panorama format. Reduces the exposure by 1 f-stop.

[neil_grant]

I have an XPAN and all 3 lenses. I use transparency film exclusively. There is no centre filter

for the 90mm - although that for the 45 will fit. I hoped I would not need to use the cf for

the 45 - but I was wrong. I use it continually - typical aperture f/8-11

[robert_ardinger]

I will add that at least in my experience if you use the 45mm in 35mm mode (non-

panoramic) the center filter is not really needed, My previous answer assumed the use of the

panorama mode.

[reisenberger]

Many thanks all for your answers. Can anyone who is leaving the centre filter on all the time (Ilkka?) vouch that they have never had _overcompensation (brighter edges) at f16-f22? (45mm lens; slide film)

 

I found results at f/16 without the center filter showed only slight fall off, and worried the center filter might overcorrect at those apertures.

 

(...Although just because the center filter graduates from 1-stop to clear from center to outer, I don't know that necessarily always translates exactly into a 1-stop to zero effect from image area centre to image area extremity - the exact effect on different parts of the image area must depend on the paths taken by the light rays to different parts of the image area, right?, which part of the filter those rays pass through and other light/lens interactions - I don't have the optics knowledge to figure this out, but if anyone does...)

 

Many thanks all

[neil_grant]

I'm sure there's no overcompensation with the centre filter on the 45mm (there would be if

it was fitted to the 90mm). If I remember rightly, from Hasselblad's original promotional

material, there is a residual (tiny) amount of fall-off EVEN WITH the centre filter.

In fact, I've never seen overcompensation of fall-off with ANY centre filter. Not with a 30mm

XPAN lens, a Schneider 90mm XL , nor a Schneider 72mm XL.

[db1]

If you look up the 45mm lens info on the hasselblad usa page, you will find " The dedicated 49 mm center filter is recommended for critical situations when transparency film is used. The center filter is normally not required with negative film if the lens is stopped down to f/8 or a smaller aperture."

[michael_briggs2]

There's two reasons for light falloff in lenses. I explained this for the 45 mm XPAN lens in response to your previous question: http://www.photo.net/bboard/q-and-a-fetch-msg?msg_id=00GLYo. The first is that the elements don't really deliver the rated maximum aperture all over the film, so the light falloff is rather severe in the corners wide-open. On this lens, this effect seems to be elmininated at about f8. The second is that all lenses with normal perspective have deliver non-uniform illumination at all apertures for several reasons, e.g., the light rays have farther to travel, the pupils becoming elliptical instead of circular.

So the center filter could still be useful at small apertures. Whether you "need" a center filter is a matter of taste, subject matter, film type (transparency vs negative), etc. It's also a tradeoff versus longer exposure times and inconvenience.

 

I think it unlikely that the center filter will over compenstate. But you can easily test. Focus the camera on infinity and point the camera towards a nearby uniform surface that is uniformly illuminated. A photo without the center photo will show you the falloff of the lens. Now add the center filter to see the result with that correction. I think it likely that you will see that there is still some falloff, as Neil predicts. This is how center filters are usually designed.

[michael_briggs2]

The Hasselblad USA page on the 45 mm XPAN lens and its center filter is http://www.hasselbladusa.com/gallery/Articles.asp?secId=467&itemId=2636. The page mostly answers your question, though it isn't entirely self-consistent.

 

First, it states "When stopping down the lens to apertures smaller than f/8, the lens is free from vignetting. However natural light fall-off (a basic law of physics) reduces the corner exposure by roughly 1 f-stop."

 

So from wide-open to f8, the lens suffers from the problem that the lens elements aren't wide enough, and it really doesn't deliver the rated aperture to the corners. The lens barrel vignets the blocks some of the light rays from reaching the corners of the image. So you will get worse falloff at wide paertures.

 

At apertures smaller than f8, such as f11 to f22, the vigentting is gone, but like all lenses of normal perspective, other optical effects remain. These will be independent of aperture, so past f8, the aperture doesn't change the light falloff. Hasselblad states that the falloff for these apertures is about 1 stop.

 

A 45 mm lens on this format actually isn't all that wide -- it is only 37.6 degrees to the corner of the 24 x 65 mm format, or 75 degrees from corner to corner. (Compare to some Schneider or Rodenstock lenses covering 110 or more degrees!) I suspect that this lens has cosine to the third illumination, which would predict about one stop falloff in the corners. (This is based on the cross-section diagram of the lens in the PDF file at http://www.hasselbladusa.com/products/level3.asp?secId=988&itemId=2706.)

But even if the lens has cosine to the fourth behavior, the predicted falloff is only 1 1/3 stops. (See http://www.photo.net/bboard/q-and-a-fetch-msg?msg_id=005gK2 for more on the theory.) So either prediction matches Hasselblad's description well enough.

 

At lot of lenses for which no photographer uses a center filter have about one stop of falloff. So probably you could easily do without the center filter at f11 and smaller apertures. But why not test to make sure?

 

Most usage of center filters for LF lenses are for small apertures, where the goal is to correct the "natural" falloff that occurs even at small apertures. In this era, LF photographers tend not to use their LF lenses near wide-open, and so the worst falloff ("vignetting") of this case isn't an issue. This Hasselblad lens isn't that wide compared to the LF lenses for which center filters are typically used. It just sometimes used in a different manner, at wider stops, where the "vignetting" comes into play. (Few 4x5 photographers use a center filter for a 90 mm lens, which has a larger angle to the corners of 4x5 film, 40 degrees, than the 45 mm Hasselblad lens does on the XPAN format.)

 

Hasselblad further states "The centre filter is optimised for larger apertures and excessive stop-down of the lens should be avoided." which fits with the idea that the main use of the center filter is for wide apertures, for the vignetting case, and even suggests that the lens not be stopped down too far when using the center filter.

 

 

But the next sentences are "With a centre filter in place, the illumination of the film is even across the whole format from corner to corner also at full aperture. With the lens stopped down to f/11, the result is extremely even.", which don't make full sense. The center filter can't make the illumination very even at both full aperture and at f11, because the illumination falloff from the lens is very different at those apertures. Wide open, we expect a lot of vignetting, while Hasselblad has told us that the vignetting is gone past f8. And the sentence before these two has suggested avoiding stopping down excessively, yet f11 and further should have the same illumination. I doubt that the illumination with the center filter is perfectly even at full aperture (as perhaps suggested by the quotation) because a 1 stop density center filter doesn't seem strong enough. If it was strong enough wide-open to fully correct, then it would over-correct at all other apertures. Probably the center filter fully corrects at f11 and beyond, and corrects well enough at wider apertures to make the results fully acceptable.

 

My conclusion is that the center filter is probably most useful from wide-open to f8. Past that it is likely that the falloff to the corner is only about one stop. That's what Hasselblad says, and what I estimate. That isn't much falloff, and can usually be ignored. Probably if you used the center filter at apertures past f8, you'd make the illumination very even.

[ilkka]

Both theory and practice seems to confirm that the centre filter can be used at all apertures, it is most useful wider open when corners can be a bikt too dark without it and even with it there is a tad of darkness, while at small apertures it makes the image illumination very even. Surely that is a good thing.

 

Sure, the 45 is not extremely wide, there is a 30 mm lens for that. But the 45 is indeed very wide, sideways about as wide as the 38mm Biogon for the square Hasselblad (for its smaller format width) and about as wide as the 43 for Mamiya 7. These are the widest non-fisheye lenses for 6x6 and larger medium format.