200mm f/4 lens focusing distance and blurry image at 1:1 magnification ratio

[Aligem]

Hello Everyone
I am new in Marco photography and I am trying to capture images of small Gemstone. I use d90 and 200mm macro lens. first of all I need to say that I use daylight in this set up so if it is wrong please let me know if I change the light and use artificial lighting, do I get the image crystal clear? I shoot at min focusing distance to keep at least 1:1 ratio of the subject on the sensor. The image and entire scene is blurry, so the problem is focusing at this minimum distance do not provide clear image of the scene and the image on the Histogram looks good and the picture is bright but not crystal clear. It is rather blurry and fuzzy than clear image.
I also need to ask you is this normal for this lens not to be able to get at least clear image at focusing distance ? or is there something wrong with my set up?
When I take a longer distance shot the image is clear and all sharp and good, so there is no problem with the lens or camera.
What if I add Microscope objective on the front of the lens? does this make the 5mm Gem large enough to be visible (((the surface of the Gem clearly and if I add strong lighting does this solve the issue )))of the blurriness of the scene? I have seen people using much smaller lens in focal length but they get sharp image and facet junctions on the Gem are more clear to visualize? can you shed some light on this to understand where the problem coming from? Your pro advice is appreciated.

 

[BF Hammer]

You write in a lot of detail in your question, but we are missing some information still. A sample of one of your photos will help so we can see the exif data and understand your exposure settings.

At 1:1 distance for a macro lens, your focal plane can be quite narrow. That means your gem in 3D space is deeper than what is possible to be in focus. It is referred to as Depth Of Field. You make DOF wider by shrinking the size of Aperture (increasing the f-stop value). This works against you in underexposing the photo. It is a compromise situation.

A tripod is a must. You will be adjusting shutter speed to such long times that you cannot hold camera steady by hand. A D90 has poor ISO low light performance compared new models also. The noise is very visible above ISO 400. Entire books are written for this kind of work.

There are also techniques for combining multiple photos in a focus-stack that can help. But this demands using a tripod so there is no movement between photos.

 

[BF Hammer]

Here is an example of manipulating Depth Of Field with your f-stop. At f/2.8 with a 90mm lens first.

And shrinking the Aperture to f/16 and rotating the subject for a flatter plane to the camera sensor.

The physics of the lenses means that my 90mm lens in the example will give a wider DOF than a 200mm lens. The more magnification you use, the shallower that DOF will get. So getting closer to your subject is going to be working against your goal of sharper photos.

 

[Clovishound]

There is a lot to consider here, but let's get the focus issue sorted first and then move on.

Like BF said, we really need to see one of the images to help diagnose the problem. I have one question. Are you getting a sharp image in the viewfinder?

One technique you could use to preserve maximum magnification is to switch the lens to manual focus. Then turn the focus ring as far as you can in the direction of closer focus. Then move the camera back and forth without touching the focus ring until the subject is in focus.

 

[Aligem]

Thank you for your massages and helpful suggestions. As Clovi said , I set the focus manually to get it as focus as possible for 1:1 ratio as closest distance and then I move back and forth on the macro rail by 1 Micrometer and the focusing blur become worse. I have done this by using M exposure mode and changed f/ at different numbers from 5.8 to 16 . The lighting changes as it should does. But the focusing has god better during course of change in settings which just made it better at the corners of the subject and I did try again but no more improvement. I have attached one image in here to see what I am speaking about. I realised by reading my book , if I change the focusing from Single to Dynamic area or centre weighted area may have better effect. I look through viewfinder and no changes , no sharp image , when I sit and change the position of the camera 45 degree to the subject it becomes more clearer image in the viewfinder so possibly there is no light enough into the sensor. But if this is the case why when I had the subject in the light box with fluorescent lighting and sitting in front of the subject with strong lighting , focusing did not happen at all? so I am going to experiment again and write in here about my findings and see your view about my process. Thank you for being friendly to respond to me.

 

[Clovishound]

It sounds to me, from your post that you are using autofocus. I would recommend that you use manual focus. You can either use the switch on the lens, or the menu item in your camera to switch to manual. It also sounds like you have a focusing rail, which is a good tool for macro work, especially studio. Looking at the image you posted, it looks like the focus point is at, or near, the back of the gemstone.

Try this: Using manual focus, turn the focus ring until you are at minimum focus distance. Use the focusing rail to move the camera back and forth until the front of the gem is in focus. Then advance until the facets about a 1/3 of the distance between the front and back of the gem are in focus. Then take your shot. If you need to reposition the tripod, because you have reached the front or back limit of the focusing rail, then reposition it.

The EXIF shows your posted image was shot at F5.6, 1/400 sec and ISO 220. I would stop down to F16 and then lower your shutter speed to whatever it takes to get to ISO 100 to 200. Since you have a stable subject and are shooting from a tripod, you can use as low a shutter speed as you need, as long as you practice good shutter technique, IE use a remote release, or self timer for long exposures, say below 1/50 sec. The smaller aperture of F16 will give you more depth of field, which allows more of your image to be in focus. If you need a little more, you can try F22 or even F32, although your image quality will suffer somewhat beyond F16. As stated in BF's post, if you need more depth of field than a small aperture gives you, it will be necessary to use focus stacking, also called focus bracketing to get everything into focus. You can cross that bridge later if necessary.

Good luck and let us know how you are doing.