# Any flickr members here?

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I've recently created a flickr's account because of my digital camera upgrade. I know a few of you are using flickr but I'm interested in knowing who exactly. So feel free to post!
My account is http://www.flickr.com/photos/mygzulis/. Out of the 27 photos I posted there, I think only 1 is not that badly taken (the one of the bird in Iguazu falls). I'm a total newbie in photography but my will to learn is great.
I noticed that I had not controled the exposure time in any of my photos, it was chosen automatically by my camera with very bad results when it comes to spiders. Previously I thought that the ISO was directly related to the exposure time but it isn't, clearly.

## Answers and Replies

Gold Member
Very nice photos Maui!
Your trips look really awesome.

ISO specifies the sensitivity of the camera, i.e. how much light is needed for a correctly exposed image.

The higher the ISO, the less light is needed. At constant aperture this means higher Iso-->shorter exposure time.

Some cameras set ISO automatically.

For a physicist: photographic aperture is a numerical aperture, i.e. the ratio of focal length to the effective diameter of the lens (diameter of the glass reduced by how much you stop down).

How much light you catch depends on the effective area=pi*diameter^2, which is proportional to the NA^2. That's why you get strange values for the aperture

1.4 (approx sqrt(2))
2
2.8 (approx 2*sqrt(2)
4
5.6 (approx 4*sqrt(2)
8
11 (approx 8*sqrt(2)
...

at each step the effective surface doubles. This matches the exposure times, which also double (or half) at each step.

So the exposure value stays constant when you close down the aperture by one step, and increase the exposure time by one step.

http://www.flickr.com/photos/carsten1968/

Gold Member
ISO specifies the sensitivity of the camera, i.e. how much light is needed for a correctly exposed image.

The higher the ISO, the less light is needed. At constant aperture this means higher Iso-->shorter exposure time.

Some cameras set ISO automatically.

For a physicist: photographic aperture is a numerical aperture, i.e. the ratio of focal length to the effective diameter of the lens (diameter of the glass reduced by how much you stop down).

How much light you catch depends on the effective area=pi*diameter^2, which is proportional to the NA^2. That's why you get strange values for the aperture

1.4 (approx sqrt(2))
2
2.8 (approx 2*sqrt(2)
4
5.6 (approx 4*sqrt(2)
8
11 (approx 8*sqrt(2)
...

at each step the effective surface doubles. This matches the exposure times, which also double (or half) at each step.

So the exposure value stays constant when you close down the aperture by one step, and increase the exposure time by one step.

http://www.flickr.com/photos/carsten1968/
When I had the Canon ixus 50, most of my photos taken indoor were blurry, not because the camera would not focus well but because the exposure time was way too high. In that sense the automatic mode was doing a poor job. By playing with the settings (I knew nothing about them at that time), one day I found out that when I chose an ISO of 400 I would never get any blur effect no matter where I took the photo. So I manually set that setting. Much later I realized that the quality of the photos taken with ISO 400 was noticeably poorer than when I chose a lower ISO (the possible values were ISO 50, 100, 200 and 400). That's basically why I thought that higher ISO = shorter exposure time.
Now I know I was totally off.
I've been playing with my new DMC-tz25. Now I can control the exposure time which very nice, especially for taking a bird in flight or anything that moves fast. I choose to auto adjust the ISO. I think I'm getting it: the shorter the exposure time, the higher the ISO needs to be for the photo not to be too dark.
By the way thanks for your explanation. If I'm not wrong, I think you made a slight mistake (effective area=pi*diameter^2 should read effective area=pi*radius^2) but I get the idea.
Also thanks for your flickr's page :)

Of course there is a price for high ISO, signal to noise ration goes down the drain. That means lot's of pea soup noise and less dynamic range and hot pixels, that turn completely white. The smaller the sensor, the worse. That's the problem with the pocket camera's.

I'm not on Flickr, but some of my pics can be found in my portfolio in my signature. Notice that the last pic of the [strike]lock[/strike] blacksmith (also in the previous photo challenge) was taken with ISO 3200.

Gold Member
Of course there is a price for high ISO, signal to noise ration goes down the drain. That means lot's of pea soup noise and less dynamic range and hot pixels, that turn completely white. The smaller the sensor, the worse. That's the problem with the pocket camera's.

I'm not on Flickr, but some of my pics can be found in my portfolio in my signature. Notice that the last pic of the [strike]lock[/strike] blacksmith (also in the previous photo challenge) was taken with ISO 3200.

Wow! What kind of camera did you use for that picture?
Also, did you/the camera choose such a high ISO because of the low light situation?

The camera is a Canon EOS 7D. I selected the ISO myself to get a good range shutter speed and aperture range (1/100 sec and f5.6) to freeze the hammering hand

Gold Member
The camera is a Canon EOS 7D. I selected the ISO myself to get a good range shutter speed and aperture range (1/100 sec and f5.6) to freeze the hammering hand

I see.
By the way your photos are absolutely wonderful. I've sneacked a bit into them, I particularly love the one of the fly over the aphids. Thanks a lot for sharing.

Thanks, happy to share the https://dl.dropbox.com/u/22026080/IMG_0214_filtered_3.jpg [Broken]. But I'm most proud of the dragonfly caught inflight. That was really difficult.

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Gold Member
Thanks, happy to share the https://dl.dropbox.com/u/22026080/IMG_0214_filtered_3.jpg [Broken]. But I'm most proud of the dragonfly caught inflight. That was really difficult.

Yeah I've no idea how you could focus on it so well and so quickly!
Pretty impressive, you could determine more or less how fast it moves its wings if you take a look at the shutter speed. I notice that its lower wings swings faster than the upper ones (at least in the picture where it moves from the left to the right).

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The credit for that incredibly fast focussing goes mainly to the aforementioned camera. It has a sort of tracking mode. Once it "locks" on a 'target' it's able to follow it and predict where it is when the next picture in the sequence is going to be taken in the burst mode of 8 frames per second. There is a reason for the price of that awesome piece of equipment.

That same principle worked for Candy on full speed.

https://dl.dropbox.com/u/22026080/candy.jpg [Broken]

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Gold Member
I think I have a similar mode in my camera, but I never used it yet. I doubt it could be as effective as yours.
1 question: when you use the 8 photos / second, is there any "loss" of quality? I mean, can you take 8 photos per second at full quality? I'm not 100% sure but for my camera I think I'd pass from 12 mega pixels to 2 mp if I do that.

Every pic is redone, both the focussing and exposure data. No loss of quality. The only issue is, that you should consider, after how many clicks the memory buffer is full, however in full high quality JPG storage, that takes 92 shots.

Notice also the shallow depth of field when using a telelens, making the focussing more challenging even. A small sensor camera is less critical with a considerably larger depth of field.

What ISO to use depends on a few factors:

- how much light is there? Very little light (like Andre's Blacksmith): High ISO. A lot of light (bright daylight outdoors): Low ISO

- Focal length: Tele is much more sensitive to camera shake (even with stabilizers) and therefore needs shorter exposure times. As a rule of thumb, the exposure time should not be longer than 1/(focal length), so for a 300mm tele, set the ISO such that the exposure time is 1/250 sec or faster.

Note that at the tele end of your zoom you are generally limited to quite small apertures (like f/5.6 or f/8). That does not leave a lot of margin for playing with aperture vs. exposure time.

- Subject movement: For moving subjects you often want to "freeze" the action. That means short exposure times. But then again sometimes you want the opposite. Take a shot, look at what you've got and change exposure time and ISO as needed.

1952 Buick Eight by carsten de, on Flickr

(Canon EOS 5D Mk2, ISO 400, f=28mm, 1/10 sec at f/5.6. This was taken at 8pm with fairly low light)

Gold Member
Ok thanks guys I understand.
By the way, M. Quack, regarding the photo of the car: I don't understand how it's possible that the back of the car seem to have moved during the exposure time of the camera while the front part has not.
I'd understand if the back of the car was out of focus while the front was right on the focus, but it doesn't look like it. It looks like the back is not only blurry but also "moving" (same for the head of the passenger). How is that possible?

Interesting question, I am not quite sure myself. It must be related to the wide angle. I was focusing at the front of the car and panning along. The car was maybe 3m away. The panning motion was a rotation, so effectively the car was rotated about the focus spot, too, by the same angle. If you are panning with a tele lens, then the angle will be much smaller, so the effect might not show up as much.

This is not some freak accident, there are a couple of other pics that show the same effect.

Probably so, I wondered the same but as the car got closer it must have 'grown' backwards. I wonder how a sequence of 6 fps would have shown, but at 1/10 shutter speed, that rate is probably not obtainable.

I'd understand if the back of the car was out of focus while the front was right on the focus, but it doesn't look like it. It looks like the back is not only blurry but also "moving" (same for the head of the passenger). How is that possible?

I would guess that it's due mainly to focus being locked only on the front of the car. Look how blurry the car by the left headlamp is. My camera has a defocus mode and can take similar pictures on purpose:

Gold Member
Hmm Maui, I don't see the similarity between your photo and M quack's.
I've been thinking about http://en.wikipedia.org/wiki/Rolling_shutter, rolling shutter. Namely the left side of the photo is scanned before the right side. If we add the focus on the front part of the car, I think it makes sense.

I don't think AF tracking is the culprit.

Assuming that the car moved at 10 km/h, it moved by ~30cm during the 1/10sec exposure time. A DOF calculator (http://www.dofmaster.com/dofjs.html) gives sharpness from 2.15m to 28.5m for 28mm at f/5.6 with focus at 4m. This easily covers the entire car, moving or not.

I don't think that this is a rolling shutter effect either. For that the exposure time is much too long. Also, remember that this is a DSLR with a real, mechanical shutter.

That leave the panning motion.

Draw the car as a box, say 2x4m, and the photographer as a point, say 2m ahead and 2m sideways. Draw a line from the point closest to the photographer, and a line from the rear of the car to the photographer.

Now displace the car forward a bit and redraw the lines. The angle between the two front lines is much larger than that between the two rear lines.

So by tracking the front of the car (rotating by the angle between the two front lines), the rear gets smeared out (by the difference in angle).

$\tan \alpha(t) = \frac{d_{\parallel}+vt}{d_{\perp}}\\ \frac{\mathrm{d}\alpha(t)}{\mathrm{d}t} = \frac{1}{1+(\frac{d_\parallel+vt}{d_{\perp}})^2} \cdot \frac{v}{d_\perp}$

For the front, $d_\parallel=2\mathrm{m}$. For the rear $d_\parallel=6\mathrm{m}$, so the sweep rate is much larger for the front (v/4m) than for the rear (v/20m). Parallel and perpendicular refer to the motion of the car.

The effect is so noticable because the length of the car is quite large compared to the distance to the photographer.

Gold Member
I think you got it here M Quack, that makes sense.
Also putting panning motion in google picture shows similar pictures. There's even a photo where the moving object's closest to the camera point is in its middle. So both the rear and front looks like "they are moving". (http://www.dailytravelphotos.com/archive/2009/12/01/)

I don't think it depends on which point of the object is closest to the camera.

What is important is that different points have different distances. I could have tried to track the rear of the car, then the front would have been smeared (in the opposite direction). Have to try this one day...