E-M1.3 vs OM-1: Differences in Focus: The E-M1.3 Does a Couple Things Better.

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drj3 Forum Pro • Posts: 12,367
E-M1.3 vs OM-1: Differences in Focus: The E-M1.3 Does a Couple Things Better.
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I am sure most Micro Four Thirds readers know about the new OM-1 and how it differs from the Olympus E-M1s/E-M5.3 when using CAF. While the OM-1 is far superior to the previous cameras in most ways, there are some differences that are important to understand to obtain the best results with the OM-1.

Based on my early use of the OM-1, I had noticed some situations where I thought the camera should have obtained focus, when it failed. So I decided to see if I could learn more about the OM-1 focus.

There is a difference in the AF Scanner.

In Mode 3 on the E-M1.3 is continues to attempt refocus when the lens fails initial focus as long as the shutter button is at half press. The AF Scanner in the OM-1 setting of ON is equivalent to the E-M1.3 AF Scanner in Mode 2 (attempts refocus once), there is no equivalent to Mode 3.

If there is a large difference between current focus and target distance and the illumination is very low or the target has little contrast, the OM-1 may fail to focus. It then will remain unfocused until you release the shutter button. It may again fail if you simply half press the shutter button again. Sh2 typically makes this more likely to occur.

In the same situation the E-M1.3 in Mode 3 will continue to attempt focus and it will almost always focus. If focus fails for the E-M1.3, then moving the camera (while half pressing the shutter button) to create contrast/detail changes in the frame will cause the E-M1.3 to the focus. Moving the OM-1 will also make it more likely to focus, but it requires a greater change in contrast/detail before it will focus.

The manuals for E-M1.3 and OM-1 indicate the camera scans the whole range from minimum to maximum focus distance when it scans. This appears to either not be completely true or, if true, then the scan can be too fast to allow focus on low contrast/low illumination targets. In my tests both cameras focused easily on the low contrast target, when the initial focus distance was closer to the actual target distance even though they failed when the initial focus distance from the target was greater.

In burst shooting, there is a difference between the E-M1.3 and OM-1 in ability to obtain focus when the target is out of focus.

This will occur in situations where the E-M1.3 and OM-1 camera may be capable of obtaining focus with a half press of the shutter button, if the burst is initiated before focus is achieved with the half press. However, it is also possible that the OM-1 may not focus on the target, since it only attempts focus a single time with the AF Scanner set to ON, while the E-M1.3 continues focus attempts with the AF Scanner set to Mode 3.

If a burst is initiated with very poor target focus, the E-M1.3 will eventually focus on the target while the OM-1 often fails. The difference between the two cameras is more obvious when using the TCs with the 300mm f4, than when using the 300mm without TCs.

For the OM-1, SSL (Sequential Shooting 20fps) is somewhat more likely to focus more or do so more quickly than Sh2 at 25fps. Sh2 at 50fps requires the initial focus to be much closer to accurate focus than SSL or Sh2 at 25fps. Setting Release Priority to OFF on the OM-1 is a safe way to avoid this issue, but may cause focus failures on small fast moving targets. Mirrorless Comparison sets the Release Priority to Off for their BIF evaluations. Using either manual or zone pre-focus is another way to avoid potential problems.

While I do not know the reason for this difference in focus during a burst, my guess is that it is related to a difference in the focus algorithms for the E-M1.3 and the OM-1. The E-M1s use focus of the previous frames to help predict focus. The previously exposed frames contain distance information which can potentially enable the camera to know the change in focus direction necessary for correct focus. So far I have discovered no evidence that the OM-1 uses prior exposed frames to improve focus prediction. The OM-1 may simply rely on the much faster and more accurate focus allowed by the faster readout speed, faster processor and more focus points to determine focus. For the above conclusions, I used a variety of different focus tests at different distances with different targets and different amount of illumination using my 300mm f4, MC14+300mm, and MC20+300mm. Results with other lenses may be different.

My first chance to do a real world test of the focus/burst performance with poorly focused targets occurred on my recent visit to the local wildlife refuge. I was using the MC20+300mm, Sh2 at 25fps and photographing birds at about 300 feet (pre-focus distance), when I noticed a very distant eagle flying toward me. I put the Dot sight on the eagle, pressed the shutter, pausing very briefly at half press (the way I might do with a flying swallow using the E-M1.3 with the MC20.) Since I wanted to test focus of an unfocused target, I used the Dot sight to keep the bird in the frame. All focus points with bird subject identification was used.

I held the shutter for 61 frames, it did not focus. I released the half press and repeated the full press of the shutter with a brief pause at half press. It did not focus. I repeated this for 217 frames without focus. I then switched to the EVF and used the snap focus ring to manually focus on the bird and continued with auto focus using the EVF as the eagle circled overhead.

In general the camera retained focus on the eagle with one exception. The focus was soft on a series of images when the eagle flew much closer, changing distance and direction. I don’t know if this indicates a potential problem when a larger focus distance between frames is needed with Sh2 or just that the combination of the MC20+300mm focuses a little too slowly at 25fps for large focus changes.

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drj3

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