Hmmph.

Also check out this thread:

https://www.pentaxuser.com/forum/viewtopic.php?t=3867

Well, Ok, fair cop.

I had a look at my 400mm F5.6 lens - 400/5.6 = 71.

I also had a look at my 200mm F4 lens - 200/4 = 50

Each of these is within a millimetre or two of the filter size - which is obviously the size of the front of the lens - and in those cases also the front element of the lens.

I should indeed have looked a bit more closely at my 50mm F 1.7, and 135mm F 3.5 lenses, each of which has a front element where the diameter of the disc through which the light passes is smaller than the diameter of the front. In the 50mm case it's 29mm - and 50/29 is 1.7. For the 135 it's about 38mm - and 135/38 is about 3.5.

So Ok, I was a bit loose in my description - in the interests of strict accuracy I should have noted that not all lenses have a front element the same size as the filter. My apologies.

I'd suggest a read of http://en.wikipedia.org/wiki/F-number

From that, it should be clear that for a given lens, the minimum F-number (which corresponds to the widest aperture) is dependent on the focal length, and the maximum size of the hole that the light gets in through.

For instance, I have a Sigma 400mm lens, designated as F5.6. This means that its minimum F-number is 5.6.

400 divided by 5.6 is 71, so it should come as no surprise that the size of the filters it takes is 72mm.

To get to F2.8, it would need to be twice the diameter - that's a big chunk of glass, so not surprisingly, long telephoto lenses with small minimum F-numbers are much more expensive, since they're much more difficult to make.

So the maximum aperture (minimum F-number) is very much dependent on the lens - no matter what you set the camera to (if the camera controls the lens aperture), you cannot get an F-number smaller than the lens's minimum.

All that said, it remains true for a given lens, that to get the shallowest depth of field, you need to set it as wide open as possible.

Be careful though. For portrait work, it's possible to get DoF so shallow, that the eyes can be sharp, but the nose and ears aren't. This may not be what you want.

Nice. I realised early that focusing during totality would be impossible, so I fiddled about quite a bit until there was about 1/3 of the moon not yet in the umbra, and then left it severely alone.

I was very, very careful not to disturb the focus when I took the TC off during totality... in the desperate (and in vain) search for sharpness

Yes, I'm kicking myself now - a minute spent doing some simple arithmetic beforehand would have made a big difference.

I was using a 400mm lens with x2 TC, that's 800mm, with an angle of view corresponding to 1200mm on a 35mm negative.

From http://en.wikipedia.org/wiki/Angle_of_view, it appears that the angle of view is 1.72 degrees.

At 360/24 = 15 degrees an hour, or 0.25 degrees per minute as someone said, in 3 seconds that's about 0.73% of my frame width.

Blown up on the screen to 24cm, that's a horizontal blur of nearly 2mm, which is obvious even with a ruler.

I should have increased the ISO to 1600 and put up with the noise - it would have been quite a bit better I suspect.

My experience too.

Pre-totality I was getting results like this:




K10D, Sigma 400mm, 1/250, F8, ISO 200

... whereas during totality they all looked like this:




I didn't move the focus in the meantime, and although I'd increased the exposure to about 4 seconds and F6.7, I was using remote release and mirror lock up. I think I can rule out camera shake - I was careful to have SR off, and I've had exposures up to 30s on the same tripod that have been very sharp.

Any ideas why the during-totality shots are so rubbish? Is 4s enough for the moon's movement to give this much blur?

I failed to win this at auction on eBay today.

http://cgi.ebay.co.uk/ws/eBayISAPI.dll?ViewItem&rd=1&item=160086419545&a...

Should I be upset? Obviously if the winner is a member here, (s)he need not reply...

Edit:

By which I mean (in case it wasn't clear), have I just missed out on a really good lens, or are such non-Pentax long zooms a bit lacking in contrast and sharpness and would I be likely to end up disappointed with it..

Thank you. Not sure I deserve the accolade, since there wasn't much rocket science to it, and most of the ones I took weren't very good, for one reason or another. I didn't want to annoy the wasp, and it wasn't all that cooperative - I had to chase it up and down the window frame several times. Shooting a moving target isn't easy when a couple of mm either way ruins the focus.

I don't mind at all.

Jessop PK/A extension tubes, with a little bit of paint scratched off where the 7th body contact fails to make electrical contact with the metal of the tube.

50mm SMC-A lens, with the 31 mm tube.

400 ISO, F22, 1/180s and the built in K10D's flash with no compensation.

Hand held, following the wasp as it wandered about. I think for some of them I had one foot on the side of the bath, and the other on the window sill. Focus was by moving the camera, not the focus ring.

In software, a little bit of cropping, adjustment of the lighting and a touch of unsharp mask to finish.

I would. Otherwise, you won't be able to control the lens's aperture from the body (unless you can set it using the aperture ring and have the camera know what you've set). I paid £50 for the Jessops PK/A tubes. No idea about macro lenses, really - ISTR a 100mm macro lens went for about £100 recently.

Most kind.


Yes, he doesn't really look evil enough, I agree. There are some wasps that I only have the courage to approach with a rolled up newspaper, that start the back of your neck tingling even with the sound of their buzz. This wasn't one of them.

Not sure quite how far into the jaws of doom I'm prepared to step for the sake of the art...

Like this one?