Episode 130: Getting the Bugs out

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Sorry, again a “Nerd Show”. I tackle some problems that have come up with the Python script of the last week and show my way to debug and test while making errors. My first error was to take a test case without colours – my script from last week changed all images to greyscale. Then I learn something useful about merging layers in Python.

But the top of the show is even more nerdy – the Octave plugin allows to access the Octave math software from GIMP. Caluculate your images or do a FFT with them.

The scripters crowd in the forum found a bug in GIMP, saulgoode reported it and it got fixed in a day. Take that, Adobe! ;-)

Then I try to explain why the big blobs in the image featured two shows ago are not made with GIMP but with pure physics in the camera. You can see a Blender video I made about this for #200 of Chris Marquards Tips from the Top Floor show. My part starts at about 11:40 and contains an embarrassing error. Do you get it?


This image by Nachbarnebenan is a good example for a shallow DoF.

The next show will be a simple, non-nerd walk through the post processing of an image. Promised.

The TOC

01:10 Octave Math Plugin
02:10 Number types in Python
06:00 Making a test case
06:30 Finding the wrong layer mode
08:00 Running into a wall and thinking around it
09:40 Isolate the bug
12:40 Cleaning up the code
14:40 Colour halos wit Normal Mode sharpening
16:40 When to use Octave sharpening
17:10 Basics of Photography – Depth of Field and Bokeh
19:00 Image creation with a lens
21:00 Circle of Confusion
22:30 Depth of Field
25:00 Apperture and DoF
28:30 DoF and sensor size
29:00 Dofmaster
30:00 Focal length and DoF

Creative Commons License
Meet the GIMP Video Podcast by Rolf Steinort and Philippe Demartin is licensed under a Creative Commons Attribution-Share Alike 3.0 Germany License.
Permissions beyond the scope of this license may be available at http://meetthegimp.org.

34 thoughts on “Episode 130: Getting the Bugs out

  1. How do you do all that math while still making good photos? I would need a calculator! Is there a trick to this or does an expensive camera can all at?

  2. Forget: I love this seagull! How you make it look like frozen? Was it sailing and not moving the feathers? And where is this? At the harbor? I was there once traveling, there are lot of seagulls.

  3. I don’t do the math while shooting – if I need a precise DoF I do the calculations beforehand with a cardboard slide rule.

    The seagull is not from me, I have updated the post above. It’s done in the Zoo in Leipzig, hundereds of Km away from the sea. The seagulls have found out about garbage dumps and other nice stuff men throws away and gone into the hinterland.

  4. Sorry for asking, but how do you do that? Do you prepare every photo even before leaving your house? This is at the night, do you use a flashlight to see your calculations? Do you remember them all? What will happen if something catches your eye? Can you do all the math in your head in a short time? Or do you have to cut corners with this?

  5. … everything except a toaster … lol

    good one, mr. teacher ;-)

    Nice part about basics again.

    I recommend everyone, who is interested in fotography to read at least one book not about software, but the old stuff concerning the camera and the lenses. This is still extremely usefull, even with the actual full automated cameras. John Hedgecoe for example is one of the good writers of such books.

  6. I second that – get some basic knowledge about the principles of photography if you want to do a bit more than snapping. And Hedgecoe is very good – but look into your second book from him carefully, he recycles a lot. ;-)

    Usually I don’t calculate DoF, and also not in this case. I used 1.8 to get the bokeh round and not pentagonal from the diaphragm blades at tried to get the LCD in focus. I knew that I would have only a shallow DoF, but was surprised of the result of the calculation during the show.

  7. I for myself think, that this would be great. What you did right with the camera, you don’t have to fix in Gimp.

    After this show I know at last, why DoF becomes wider with bigger apertures. Of course I knew the fact, but I couldn’t explain, why. Some shorties from time to time wouldn’t bother the only in software interrested viewers to much I guess.

    You are right with Hedgcoe. So public libraries are not a bad idea at all. And because the old stuff doesn’t become out-of-date so fast, you could buy although used older books.

  8. Thanks for answering the programming questions in a way that even I could understand you make it all look too easy. It is a very good idea to cover the physics involved in photography which is as valid today as it was when the camera obscura was first invented. Your almost throw away comment about DoF changing with sensor size is quite important and another reason for moving on from simple point and press.

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  10. Another great episode. I was enjoying the tech talk, it all seemed like old news but it was fun watching
    the very creative way Rolf uses his ‘blackboard’. And then suddenly he explained ‘Circles of Confusion’.

    This was a term I had heard before, but never understood. So once again, going over the basics is again
    a good thing.

    I will second that about the 50mm 1.8 lenses. They are so cheap that if you buy just a body, this should be
    the very first lens you buy. Very fast too! Not the finest ‘bokeh’ but still very good for the price.

    After ISO and any in-camera profiles [not needed for RAW], the next two decisions are aperture and
    shutter speed. Most P&S will be set up to handle both in full Auto mode. Under Manual mode, both must
    be set. With a ‘bridge’ camera or better, shutter speed can be set for Shutter priority mode. But most use
    Aperture priority, so they set Aperture and let the camera do the rest [this gives the most control]. This
    usually gives a shallow DoF and can make for a nice effect. Almost all action shots will be taken this way,
    although some may need full Manual. A shallow DoF also draws the eye towards the subject, so once
    again a good thing. Adding a background is a part of composition and is nice to do – if you have got
    the shot you wanted first. Otherwise a blurry shot with a nice background is worthless.

  11. Today I try to shot several photos in the first snow. We had a fox with its young in our backyard and I tried to capture it. It was so sweet! But the photos are all almost black. They all look fine on the camera later and as I looked at the camera lcd. But in Gimp they are black. I can raise the levels until the photo is total white but few is there. Like taking a photo at darkest of night and brighten it. At some photos the flash activated but they are as black as the others.
    At first my camera seems alright, I tried it indoors and the photos are nornal. But I tried to shoot my daughter in the snow and all is black again. No I am afraid it is broken. Is there a way to tell? It seems working and fine and the photo on the lcd looks as wanted but the result is too dark. I tried another memory card but get the same. If the camera is broken, why are indoor photos still normal?

  12. Try another program to look at your images, even Firefox can do that. Perhaps you have a misconfigured GIMP. If not,

    Check all settings of your camera with looking into manual or do a full “factory reset”. You may have changed your setup.

  13. Thank You for the advice. I did a full reset already. I dont understand it. I take photos inside the house or at the office and they are normal. Even with the flash. But with lots of snow they get dark. I also tried at a park near my work. Maybe it is too bright for the camera? I tried all the scene modes and only back-lit gives a full white photo. It is white on the camera too. And it is not Gimp at fault, Firefox and Opera give identical results.

  14. Is it possible my camera has no function for this? I dont have an expensive like you. I can only choose auto scene or one scene mode. If I understand correctly I need to make the camera think it is dark outside and get more light to the photo? I tried back-lit, night and dawn whose are recommend for dark light but it does not work. I read the manual but I can only change the lcd brightness not the photo. Is there another way to do this or does Santa has to squeeze a new camera into his bag?

  15. I would bet it has that function, every digital camera I had in my hands had it. Which model do you have?

    Get your manual aout and look for “exposure compensation”, “exposure correction” or something like that. The usual logo on the button is a square with a +/- in it.

    You aer right, the camera has to think that it is darker, because it’s darker than the camera thinks. ;-)

  16. I use a kodak z1485. I found it in the manual. It is missing the hungarian translation so I looked for the english manual pdf on the cds. It works as the camera is selected to p mode. The function is called brightness trimming. I can chose from +2.0 up to -2.0. As I set everything except 0.0 the display goes off. Only the numbers and time and remaining battery is seen but no picture. I can take photo and it is a little less dark in Gimp but all grainy with noise. And without picture I cant see what to photograph. I tried looking in normal mode but as switching to p mode reset the zoom everytime.

  17. “You aer right, the camera has to think that it is darker, because it’s darker than the camera thinks. ;-)”

    Correction: In snow the camera needs to know that it’s brighter than medium grey. So you have to set the exposure compensation to positive values (as +1 or +2/3).

    @ bubblegum: I would go to the next electronics shop (or wherever you have bought your camera) and let it check there. It’s quite usual to get dark images in bright situations as snowy landscapes. But it’s definitely unusual to not see any picture on the screen when using exposure compensation.

    An addition to the basic photography books: Lately, I have bought a really old book in our library for just 1 EUR. It contains approx. 200 tables for DOF – at different focal lengths, apertures, camera-subjest distances etc. Additionally, you find tables for the right exposure settings and other such stuff. Very cool!

  18. The shop assistant told me, all camera of this series are like this. Replacing the lcd image by a histogram in p and m mode. I decided to buy a new camera. She gave me a whole refund and it is a year old. I cant afford a good camera like Rolf and I bought a D90 by Nikon. At home I am surprised. I did not expect it to be that heavy! I can hardly carry it around for long times. Lucky it has a strap to carry. The manual is english only and as large as an telephone register! I hope I can remember all.

  19. Why should the Nikon D90 not be a good camera? I think, it’s one of the best. In fact, I wish I would have one ;-) The D300 is much heavier by the way.

    Maybe you can download a manual in your language at the Nikon homepage. But you have to register your camera as far as I know.

  20. At first she suggested a D300s at the shop. I wanted to buy it because it is not as expensive as I am afraid. But it is big and heavy. I cant hold it in front of my eye long to see what to photograph. And it is too large for my hands. I cant use the dial or buttons. She said D90 is the only one smaller she has with good quality. But it is still big. And the lens try to topple it to the front. It needs lot of strength. Photographers assume are only big strong men like Rolf with large hands.

  21. The D90 is a very good camera – I had a D70 until my wife gave me the D200. You will get used to the weight and size soon. It is a different way to take photographs than with a Point&Shoot.

    What lens do you have on it?

  22. Attached lens is an “Tanron AF 70-200mm 1/2.8 (1F) Macro” by the name printed on. She suggested I buy two different lens. But the second one will be delivered. Both are not as expensive as other lens in the shop. But she said it gives good quality.
    You are right I will get used to it. I already took some photos today. Very nice thing to photograph vertical photos. The camera has second button. A little hard to reach but more simpler as twisting your arm. I will upload photos when my dsl at home is working again.
    There is no good manual in hungarian. Even I can tell the translation is not good. She gave me the english one too without cost.

  23. 70-200 is a good tele zoom range, but not the every day walk around lens. And now I understand your comment about “heavy”.

    I beg to differ on bigstar’s lens recommendations. Good zooms with 10x and more are really expensive. Nikons 18-70 was my every day lens until I got the 18-200 VR (which costs a real lot of money). 18-55 is another good and not expensive range. Changing the lens is not a problem and the gap between 55 and 70 can be closed by walking or cropping.

    Wait before you buy something else – and if you have a bit of money left, get the 50mm/1.8. Can be second hand, but “photography starts there”.

  24. The other lens arrived today. It is an “SP 17-50mm F/2.8″ by the same Manufacturer. I like it as is not as heavy. Switching the lens is scary. I can see inside the camera if the lens is not attached. Is this usual? Thank You, I will ask about the lens you wrote. Tomorrow is the last day of selling.
    I tried to shoot outside today and it works much better unlike my previous camera. I use the old memory card but I have to buy a new. Now it only holds 300 photos before it stores more as 4000. The camera is quick, if I push the button it makes photo without stopping unless I let go. It is not bad, it is difficult to hold the camera. I hope one photo is good, the other to remove.

  25. You don”t have to be scared by switching lenses, it is built for that. Don’t touch the internals and don’t let the camera lying around open so that dust gets in.

    Thom Hogan says that your lens is a good choice: http://www.bythom.com/1750lens.htm

    2.8 is also a lot of light and shallow DoF. Don’t look for the 50/1.8 just now.

  26. I loved this episode mainly because I love photography and im learning all the time I understand most concepts but seeing it put in pictures like that was great :) and only helped to make me fully undertand circle of confusion so thanks for that.

    On the point of Depth of field distance from background and magnification also help a great deal I have found. So if you want to decrease the DOF use your highest aperture lowest F-Number and move your subject further from the back ground and you can also move your self further from the subject and zoom a little (I think thats right).

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Anything to add from your side of the computer?