Flange Back

[Marty Hudzik]

I am assuming this does not apply to the manual lens in any way since it has a physical flangeback adjustment. Right?

[Steve Rosen]

It has always driven me crazy that you have to adjust for back focus with video cameras.. you don't have to with film cameras.. but, because I've been "warned" I do it regularly and with precision...

To tell the truth, I've never tried just putting the lens back on the camera, as I would do with my 16mm camera (for which I have 7 zoom lenses and 5 primes)...

Could someone explain what changes when you remove and then re-mount a lens on a video camera?

[Christopher Glaeser]

Originally Posted by Steve Rosen

It has always driven me crazy that you have to adjust for back focus with video cameras.. you don't have to with film cameras.

Isn't it the reverse? Digicam owners are driven crazy each time they have to send their camera to a repair center to match a body to a new lens. Video camera owners should coun't their blessings they have the ability to match a camera and lens in the field.

Best,
Christopher

[Steve Rosen]

I'm not sure I understand your response, Christopher -

Let me just say, rather than argue "abstract concepts", that I personally bought my first 16mm camera, a Bolex, in 1962 not far from where you are - Los Altos Rancho Camera Shop - I bought it off the shelf with an early Pan-Cinor zoom. By 1973 I had owned 3 other Bolexes and an Arri 16s and and Arri II (35mm).

For example, I shot a feature length documentary (ACAPULCO GOLD) during that time with a 400' Bolex and an Angenieux 12-120 and a Switar 10mm - it was blown up to 35 and distributed nationally - and it's currently available on DVD (although I don't get a dime from it).

During the 70's I owned a super16 Eclair NPR, an Eclair Camerette and a CP16. During the 80s I owned a total of five Eclair ACLs (one super16) and in 1990 bought a used super16 Aaton LTR7, which I still own.

As a result, I have owned nearly 20 zoom lenses made by Angeniux, Cooke, Century Optics, Zeis and Canon, as well as numerous primes.

Except for having the back focus CHECKED (just to be sure) when buying a new lens, I have never had any camera or lens ADJUSTED for back focus - You buy them, put them on, and shoot... That has been my experience..

So, what I'm asking is, simple question that has never been answered to my satisfaction - "why do they advise that you redo backfocus adjustment every time you change a lens on a video camera? - what changes?"

I do it because I'm careful, and because it says to do it, but "why?"

[Chris Hurd]

Originally Posted by Steve Rosen

why do they advise that you redo backfocus adjustment every time you change a lens on a video camera?

I'm not sure if I'm the culprit here or not... please see my post a couple of replies above. What I meant was that an XL H1 owner should only have to adjust FB one time only with each lens they have, just so the FB memory presets are in the camera. I certainly did not mean to imply that it should be done each and every time an XL lens is changed. This is my second apology for the miswording of that statement; I have a feeling that several more will be called for before this non-issue is finally buried for good.

[Christopher Glaeser]

Originally Posted by Steve Rosen

Except for having the back focus CHECKED (just to be sure) when buying a new lens, I have never had any camera or lens ADJUSTED for back focus - You buy them, put them on, and shoot... That has been my experience.

So, you agree the lens and digicam body should be CHECKED for focus. What if the lens does not match the body and you purchased the lens mail order? A search of dpreview.com will show that it is not uncommom practice for owners to send their body and collection of lenses to a repair center for calibration.

The only point I was making is how nice it is that you can do this calibration yourself with the XL H1.

Best,
Christopher

[Nick Hiltgen]

::edit::: clicked on dpreview and realized by digicam's christopher was talking about digital still camera's not digibeta cameras (which also go in for lens matching) so my post was a waste of space....

[Steve Rosen]

Please understand, my comment was meant as a general question about video cameras, not a comment on the H1 itself.. the flange back memory function is a terrific tool..

Also, yes I always check a 16mm lens when I buy it, but I only check it when I buy it, not every time I remove then replace the lens... And I've never had to have one modified or adjusted, even wide angle zooms, because I always buy lenses that are in pristine condition, or new ...

It just suddenly struck me as curious that I have always accepted (and, I admit, recommended, as on this forum) doing it on video cameras - but I have never questioned why, that's all... My guess is that it needs to be done because the lenses ARE adjustable, and therefore can inadvertably be changed... but I just don't get why they have to be adjustable in the first place with modern non-tube cameras..

[Christopher Glaeser]

Originally Posted by Steve Rosen

but I only check it when I buy it, not every time I remove then replace the lens.

Somewhere I think you may have misunderstood this "every time" requirement. You don't need to calibrate SLR digicams OR the XLH1 every time a lens is changed. I'm not sure where you picked up this "every time", but it just isn't needed. In addition, the nice thing about the XLH1 is that you can do the lens calibration yourself. Not so with the Canon SLR digicams. If one or more lenses need calibration, you have to box them all up and send them to Canon repair service. That's why, when you said you were "driven crazy" with the XLH1 and then went on to compare it to still cameras, I thought perhaps, just perhaps, you had misunderstood the benefits of the XLH1. Perhaps I'm wrong, and perhaps someone else can step in and assist, but when you continue to use phrases liike "every time", I think you may still be misunderstanding this issue. Hoping that helps.

Best,
Christopher

[Johan Forssblad]

Hi, I'm just curious, what is actually changed and how when the back focus of the XL H1 is changed? (I know Hasselblad adjusted the length of their still camera bodys with a big hammer (!) if the body was too long. First they replaced the lens with a metal cylinder. Then they adjusted the length by banging with the hammer until the length was correct!)

[Steve Rosen]

Christopher: Every manual I've ever read recommends readjusting back focus every time you remove or change a lens on a video camera... And, as I said, I do it because I'm careful, and recommend the same procedure to others... Better safe than sorry, but still, seems unnecessary somehow...

[Marty Hudzik]

Originally Posted by Johan Forssblad

Hi, I'm just curious, what is actually changed and how when the back focus of the XL H1 is changed? (I know Hasselblad adjusted the length of their still camera bodys with a big hammer (!) if the body was too long. First they replaced the lens with a metal cylinder. Then they adjusted the length by banging with the hammer until the length was correct!)

I'm pretty sure you are micro adjusting the distance from the back element of the lens to the CCD. If for example it is supposed to be 6.5mm but in reality, because of tolerances in manufacturing process, it is 6.4mm, you can adjust it back to 6.5mm. On the manual lens you actually mechanically move the back element of the lens. With the servo lenses I think you are moving the CCD to compensate (controlled electronically inside the XL-H1 when you do FB adjustment). I am not sure. But somehow you need to get them to the proper distance.

As a note, I randomly picked 6.5 mm......I have no idea what realistic measurements are.....this is just theoretical.

Peace!

[Christopher Glaeser]

Originally Posted by Steve Rosen

Christopher: Every manual I've ever read recommends readjusting back focus every time you remove or change a lens on a video camera.

Video cameras that do not have the ability to remember the FB setting for each lens may require a readjustment with each lens change. The XLH1 stores the FB settings for each lens to memory, so in theory (and in practice according to some XLH1 owners), a readjustment is not necessary with each lens change, but only the first time each lens is attached.

If you see an actual difference in your XLH1 FB setting each time you reattach a lens that has already been FB set, let us know.

Best,
Christopher

[Craig Chartier]

Well there are several reasons to backfocus video lenes. For one thing remember your COC charts for film. larger circles for larger film formats. Also take into account that when light hits a frame of film it penetrates all the way to the back of the film.causing a chemical reaction. light must hit the front of a pixel for the image to be in focus> film thickness is massive compared to the surface of a pixel. video mounts are made of soft medals that actually expand and contract with temp changes. We will backfocus HD cams several times a day if the temps change. Film cameras do have shims placed behind the mount to correct for manufacturing tolerances being off. PL mounts are often reset or changed out as they wear on film cameras.

[Nick Hiltgen]

Adding to what craig said, not only do the temperature changes externally come into play but internally as well, I'm pretty sure there aren't too many film cameras with internal fans to regulate the temperature, this is common in the video counterparts.